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Acclimatising your fish

Acclimating fish to your water conditions

Whether you’re introducing fish to your main tank or a quarantine tank, the basic process should always be the same: expose the fish to as gradual a change in water chemistry as possible. A few fish have evolved to handle big changes in water chemistry (brackish water fishes, for example) but most react very badly to sudden changes in pH, hardness, salinity, and temperature. Whether the fish is going from worse to better water conditions is irrelevant here, so don’t assume the fish will be happy to get out of the dirty water in the bag and into a proper aquarium because it doesn’t work quite like that. Fish adapt over time to even bad conditions, and will need to re-adapt to the better conditions in your aquarium.

The traditional way to introduce new fish into the aquarium is to place the bag containing the new fish into the aquarium unopened and leave it floating for 20-30 minutes so that the temperature inside the bag matches that of the water in the aquarium. The bag can then be opened and the fish gently poured into the aquarium. The problem with this method is that while it eliminates the problems of temperature change, it does nothing about water chemistry. So a common solution is to use a needle to pierce the unopened bag half a dozen times while it is floating it in the tank. Water will diffuse in and out of the bag, gently changing the water chemistry inside the bag to that of the water in the aquarium. This approach can work very well, but the downside is that water from the bag gets into the aquarium, dumping the load of ammonium that accumulated while the fish was being transported and potentially allowing in any swimming parasites that were in the retailer’s tanks, such as whitespot.

A cleaner method is to half-fill a bucket with water from the aquarium and float the pierced bag in that. This will allow the fish to adapt to changes in water chemistry and temperature safely, while keeping the dirty water in the bag isolated from the aquarium itself. After acclimation, the fish can then be netted out and put in the aquarium, and the water in the bucket discarded.

Some aquatic organisms are more sensitive to water chemistry changes than others, and for the most delicate species, particularly marine fish and invertebrates, the "drip method" is particularly favoured. If you are shopping for fishes likely to need this technique, it’s a good idea to ask your retailer to pack them in an oversized bag with as much water as possible. This is because you will pour the contents of the bag -- water and livestock -- into a clean bucket, and if there isn’t enough water in there, the livestock may be exposed or unable to swim properly. If there isn’t enough water in the bag, use the pierced-bag technique to add the minimum amount of aquarium water to the bucket to safely cover the livestock. Either way, release the livestock into the bucket and discard the bag. The trick now is to drip in tiny amounts of water over a long period so that the water chemistry gradually changes over time. Airline tubing and plastic regulator valves work perfectly and are very inexpensive and can be easily bought at any aquarium shop. You’ll also need some type of attachment device to hold the tubing onto the tank; again, inexpensive clips and suckers can be purchased to do the job, or else DIY solutions using adhesive tape will work just as well. Either way, use the tubing as a siphon, placing the valve at the end to control the flow so that nothing more than a drip at a time comes out. When the water in the bucket has doubled, scoop out a cupful and throw it way, and then allow the level to rise again. Do this once or twice more. The whole process should probably take about an hour, and in that time the fishes or invertebrates will have slowly and safely acclimated to the new conditions. They can then be carefully removed from the bucket and put in the aquarium.

A word about moving fish and invertebrates. Most fishes are fine exposed to air for a short while, but pufferfish can pump themselves up with air and find themselves unable to deflate themselves properly, and many invertebrates, such as sponges and sea urchins will die if bubbles of air get trapped inside their bodies. When moving such delicate animals, scoop them out of the bucket using a suitably sized container that keeps them underwater at all times. A clean jam jar or ice cream carton is ideal for this. Using plastic or glass containers instead of nets also works very well with spiny fish such as catfish that tend to get tangled up in nets. Damage to the skin following attempts to untangle them can make them vulnerable to fungal and bacterial infections. Also bear in mind that some fish and invertebrates have stings, so when transporting them, take special care to avoid touching them. Among the species known to have stings are lionfish, stonefish, waspfish, scats, numerous catfish, some sea urchins, and certain corals. Surgeonfish and spiny eels aren’t venomous but they are equipped with sharp spines they will readily use when stressed, so handle them with care as well.


On the whole plants tolerate changes in water chemistry very well, but almost always being uprooted causes some degree of shock, and most plants will shed a few leaves after they are planted in your aquarium. It usually takes a few weeks for plants to settle down, but assuming the conditions are within their range of tolerances, most plants recover from the ordeal of being uprooted and transplanted without problems. The chief exceptions are among the marine algae: most need to be acclimated to a new aquarium in the same way as delicate marine invertebrates, using the drip method. Further exceptions are among the mangroves. While most mangrove propagules ("pods") can be transported easily between fresh, brackish, or salt water conditions, once the propagule has developed leaves or roots it must be kept at whatever water conditions it was germinated under, so a salt water adapted mangrove plant can’t be added to a brackish water vivarium, or vice versa.

The Nitrogen Cycle

The Nitrogen Cycle

Successful fishkeeping is all about managing the nitrogen cycle; get that right, and everything else will fall into place. The vast majority of premature fish deaths are caused by faulty management of the nitrogen cycle. Besides being toxic in themselves, nitrite and ammonia weaken the ability of fish to combat opportunistic infections. As a result, when water quality declines, problems with fungus, finrot, and whitespot increase. In this article we’ll review the principles behind biological and chemical filtration, the two main methods for removing from the water the nitrogenous wastes produced by the livestock, and look at how each can be best used in the aquarium.

Where do the nitrogenous wastes come from?

Ammonia is the end product of protein metabolism. It is toxic, so all animals have methods for getting rid of ammonia as quickly as possible. Fish mostly get rid of ammonia through their gills. Bony fish excrete the ammonia directly, whereas cartilaginous fish (such as stingrays) produce urea instead. Either way, the ammonia gradually accumulates in the aquarium until it reaches toxic levels, at which point the fish will die. Two factors affect the toxicity of ammonia: pH and temperature. Ammonia becomes more toxic as pH and temperature rises. For this reason, fish in marine aquaria or tanks containing African Rift Valley lake conditions are particularly sensitive to high ammonia concentrations.

One misunderstanding newcomers to the hobby have is between the visible wastes on the bottom of the tank and the invisible wastes in the water. For the most part, the solid wastes are faeces and decaying plant material, neither of which contain much ammonia. Though unsightly, these are not a threat to your fish and can be siphoned away easily at each water change. Paradoxically perhaps, a spotlessly clean aquarium can be a very hostile environment for your fish if the ammonia levels are high, while an established tank with good filtration will be a much healthier place even if it looks a bit messy at first glance.

Chemical filtration

The simplest and quickest solution is removing the ammonia directly using an ammonia-absorbing chemical such as zeolite. Zeolite is inexpensive and effective, and works as well in simple bubble-up box filters as it does in high-pressure canister filters. Although it only works for a few weeks before it 'fills up' with ammonia, it can be easily 'recharged' by a good soak in salty water. Unlike biological filters it works at pH 6.0 and less, making it the ideal choice for use in acid water aquaria. Zeolite doesn't need a running-in period either, so a chemical filter can be pressed into use at a moment's notice, making it ideal for use in quarantine, breeding, and hospital tanks. Another advantage of chemical filtration in hospital tanks is that zeolite is not affected by medications or antibiotics, some of which can kill biological filters.

Sounds perfect, right? Zeolite does have its limitations. For a start, although recharging works a few times, eventually the particles become so covered with gunk and bacterial slime that the only option is to throw it away and buy some more. Admittedly, zeolite doesn't cost much, but over the lifetime of an aquarium, replacing the zeolite every few months will need to be factored in. This becomes more tiresome and more expensive the bigger the tank and the messier the fish; while zeolite might be a viable approach for a tank containing a few killifish or a breeding pair of chocolate gouramis, it is much less attractive to an aquarist with a tank full of predatory catfish or Oscars. Standard-issue zeolite is also less effective in salty water than in freshwater, preferring to take up other minerals instead. While there are marine-grade ammonia-absorbers, these are less commonly sold and rather more expensive. The biggest problem with zeolite is that you cannot tell when it is "used up", and at-least weekly ammonia tests are essential to establish the frequency with which the zeolite in your filter will need to be replaced.

Biological filtration

Compared with chemical filtration, biological filtration is cheaper and far easier to maintain over the long run, but more fiddly and temperamental to set up in the first place. Biological filters come in a variety of styles, from the classic undergravel filter through to high-tech wet-and-dry trickle filters, but all work in the same basic way. Water is passed through a porous medium that hosts a large population of bacteria. These bacteria convert the ammonia into less toxic nitrite and then relatively non-toxic nitrate. The nitrate is removed, or more specifically diluted, with each water change, keeping it at a safe level. The microbiology of aquarium filtration is not all that well understood. It is certain that the bacteria converting ammonia to nitrite are different to those converting nitrite to nitrate, but the identity of the species involved is still being studied. Bacteria of the genera Nitrosomonas, Nitrosospira, and Nitrosococcus appear to be involved with the oxidation of ammonia to nitrite, while Nitrobacter bacteria are the ones oxidising nitrite to nitrate.

The differences in bacteria in filters adapted to freshwater, brackish water, and marine aquaria are unknown. As a rule, bacteria from filters matured under one set of conditions cannot be transferred to another. While a freshwater aquarium can be converted to a low-salinity brackish water one (where the SG is 1.003-1.005) without any loss of biological filtration, when the salinity is raised much above this level the filter destabilises and needs to be cycled again. High-end brackish water filter bacteria (SG 1.010 upwards) seem to adapt readily to marine conditions, and vice versa. This seems to suggest saltwater filter bacteria are more tolerant of salinity changes than freshwater ones, but detailed studies are lacking.

Cloning filter bacteria

Regardless of the composition of the bacteria in the filter, they have to get there from somewhere. By far the easiest way to do this is simply take some filter media out of an established filter and put it into the new filter. This process -- called cloning -- kick-starts the colonisation of the rest of the filter media in the filter and should result in a stable aquarium within days. Provided the tank is not overstocked and the fish not overfed, an established aquarium can lose half of its biological filter media and remain healthy, the remaining bacteria will colonise the clean replacement media almost at once, rapidly building up their numbers as they take advantage of the ammonia and nitrite in the water. Conversely, a new filter half-filled with old media from another tank is almost as good as an established filter, and the new tank will certainly be safe enough to carry a moderate loading of aquarium fish.

Adding "living rock" to a marine aquarium is similar in effect to cloning a filter. The living rock brings in a load of bacteria along with all the other life forms that infest them, and these bacteria will quickly spread to the filter. Good-quality living rock itself has a substantial ability to perform as a biological filter medium itself, and for this reason many marine aquarists mature their tanks by installing large amounts of living rock.

Cycling with fish

If you don't have another filter from which to grab some established filter media, you'll need to cycle the aquarium instead. The required bacteria are on plants and in the air and water, and will multiply rapidly in an aquarium provided they are given some 'food'. The stuff they need is ammonia, and that has to come from somewhere. Cycling with fish essentially means putting in a few, very hardy species to act as ammonia donors. In the process, these fish are exposed to dangerously high levels of ammonia and nitrite. These fish can and do end up being "sacrificed" if they cannot tolerate these conditions, and for this reason cycling with fish has begun to fall out of use. That said, if done properly it can work and the fish needn't end up dying if only the hardiest species are chosen. Zebra Danios (in freshwater aquaria) and black mollies (in marine or brackish aquaria) have generally proven to be the best species for cycling tanks, showing little apparent discomfort from elevated levels of ammonia or nitrite provided substantial (50%) water changes are regularly performed (i.e, at least weekly).

Besides fish, cycling also requires an ammonia or nitrite test kit. Many aquarists use both. With the first fish installed, the ammonia levels will rapidly rise, followed shortly afterwards by the nitrite levels. The ammonia comes from the fish of course, but the nitrite indicates that the bacteria are starting to convert the ammonia into nitrite. As they do so, the ammonia levels will start to decline. Once there is nitrite in the water, the bacteria that convert nitrite to nitrate will start to appear, and the nitrite concentration will begin to decline. After about four to six weeks, the ammonia and nitrite levels should be close to zero, and you can add a few more fish. The filter bacteria will more quickly ramp up their efforts this time, and any spikes in ammonia or nitrite levels will be short-lived. As the weeks pass, you can add more fish, and the aquarium will be fully cycled within 2-3 months in the case of a freshwater tank, and anything up to 6 months for marine aquaria.

Fishless cycling

To avoid using fish as the ammonia source, many aquarists have added ammonia directly to the water. The bacteria don't care where the ammonia comes from, and such an approach can work well. Ammonia is nasty stuff though, and should be kept away from pets and children. The ammonia is added to the water until a reasonably high concentration around 5 mg/l) is attained. Because bottles of ammonia vary in concentration, this is very much a process of trial and error: add some ammonia, let the water get pushed around by the filter for ten minutes, do a test with the ammonia test kit, and add more ammonia if required. Make a note of how much you added to get to the desired concentration. Repeat this dose each day. The basic result will be similar to that of cycling with fish: a couple of weeks of high ammonia levels, then a few weeks of high nitrite levels, and then everything will settle down. Once you're happy the tank has cycled (no ammonia or nitrite) you can add some hardy fish, safe in the knowledge that the filter is working properly and the fish will not be stressed.

Optimal water conditions

The ideal water conditions for all fish are these: zero ammonia, zero nitrite, and nitrates that are as close to zero as possible. In an established aquarium, ammonia and nitrite should not be a problem, but nitrates can be. The water coming out of the tap may well have nitrate levels above the tolerances of certain freshwater aquarium fish and well above those preferred by marine fish and invertebrates. Thames Water, for example, aims for a standard nitrate concentration of 50 mg/l, well above that which is acceptable in a properly-run reef tank. Nitrate-removing chemical filter media are available but these are more for "polishing" essentially nitrate-free water than bulk removal of nitrate, and the nitrate-removing biological filters are cumbersome and expensive since they need to create anaerobic conditions for the required variety of bacteria. For all practical purposes, adding nitrate-free water during water changes is the preferred option for aquarists keeping nitrate-intolerant species. Reverse-osmosis filters are the best tools for this, producing about 1 litre of nitrate- and mineral-free water for every 10 litres of regular tap water. This water can be hardened as required with tap water or freshwater "salts" for use in freshwater tanks or turned into brackish or salt water by adding marine salt mix. A cheaper, but less secure, way to obtain relatively clean water is to collect rainwater.

Although not really suitable for use in marine tanks because of the leaf litter and other gunk that collects in the gutters and water butts, once strained and filtered rainwater works very well with freshwater fish. Again, it may need to be mixed with tap water to get the right pH and hardness, and filtering through carbon will remove any unwanted organic compounds, but rainwater is at least plentiful in the UK, and a rainwater butt is cheap to buy from any garden centre.

Even if you are stuck with using tap water, high nitrates are not too serious a problem. Ordinary community tank tropicals are fine at 50-100 mg/l, and adding vigorously growing plants to a tank will reduce the nitrate levels to some degree. Performing at least weekly water changes of 50% will keep the nitrates low enough that the fishes will be happy and healthy. Though often overlooked in favour of fancy equipment and chemical additives, the value of regular water changes cannot be overstated. Proper filtration and generous, regular water changes are the keys to managing the nitrogen cycle in the aquarium, and together form the royal road to successful fishkeeping.

Buying Fish

How to buy fish

Choosing the right fish for your aquarium is one of the most important things you can do to make the hobby easier and more pleasurable.

The vast majority of problems stem from poor choices at the shopping stage of the operation rather than anything the aquarist does later on. For example, buying too many fish at once is a very common mistake. In a newly set-up aquarium especially, adding too many fish at once overwhelms the biological filter leading to a sharp drop in water quality. This in turn stresses the fish, making them more sensitive to opportunistic infections such as finrot. If conditions don’t improve quickly, poor water quality will also kill fish outright, by simply poisoning them.

In this article, we’ll look at the key factors you need to consider before buying fish. To start with, let’s consider the most important factor of all: experience.


Fish can be divided up into three basic types as far as experience goes: species that are good for beginners; species that are not especially difficult but for one reason or another are best suited to hobbyists with at least some experience; and fishes that are difficult to keep and are best left to advanced hobbyists.

Fishes that are ideal for beginners tend to share a number of features in common. Most are relatively small, so do well in average-sized aquaria. They generally eat a variety of foods including flake, so aren’t difficult to feed. They aren’t fussy about water chemistry and usually adapt well to anything supplied by your local water board. Finally, they are hardy enough to live through the early stage in a hobbyist’s career where occasional problems with water quality might be a problem.

None of these attributes mean that these fish actually thrive on neglect and poor maintenance, but if you make the odd mistake, these fish should come through without much harm. Classic beginners’ fish include:

X-ray tetra, Pristella maxillaris
Bloodfin tetra, Aphyocharax anisitsi
Buenos Aires tetra, Hyphessobrycon anisitsi
Spanner barb, Puntius lateristriga
Zebra and pearl danios, Danio rerio and Danio albolineatus
Peppered and bronze catfish, Corydoras paleatus and Corydoras aeneus
Bristlenose catfish, Ancistrus spp.
Australian rainbowfish, Melanotaenia spp.
Gold and blue gouramis, Trichogaster trichopterus
Thick-lip and banded gouramis, Colisa labiosus and Colisa fasciata

A step up the difficulty ladder are fishes that place extra demands on the aquarist in terms of care. While not intrinsically delicate or difficult to keep, if these demands aren’t met, disaster often follows. Neon tetras, cardinal tetras, tiger barbs, red-tail sharks, goldfish, common plecs, guppies, mollies, dwarf gouramis, and virtually all cichlids including angelfish fall into this category.

Neons and cardinals, for example, are somewhat sensitive to poor water quality, and quickly die in immature aquaria. They are also so small that many larger fishes (such as angelfish) simply view them as food.

Tiger barbs are hardy and adaptable, but they are notorious fin-nippers, especially when kept in too-small a group. Red-tail sharks are certainly spectacular fish, but they are incredibly territorial and highly aggressive, so need lots of space and are strictly one-to-a-tank animals.

Goldfish are big, schooling fish that shouldn’t be kept in small aquaria. There’s a good argument for simply calling standard goldfish pond fish and leaving it at that, while reserving fancy goldfish for a large, well-filtered subtropical aquarium.

Common plecs are placid and reliable animals, but their large size (typically over 30 cm) makes them unsuitable for tanks less than 200 litres in size.

Wild guppies are very hardy and make excellent fish for beginners, provided they are kept in hard and alkaline water conditions, but fancy guppies are delicate and disease prone, especially when kept with nippy tankmates such as tetras and barbs.

Mollies are very hardy in brackish or saltwater conditions, but in freshwater tanks tend to be delicate and are best avoided by beginners. Male mollies can also be rather aggressive, and need to be kept in a large tank.

Dwarf gouramis were hardy once upon a time, but in recent years Dwarf Gourami Disease has become a major problem with this species. This disease is highly contagious and essentially incurable. Unless you can source locally-bred dwarf gouramis, this species is best avoided.

Cichlids are so diverse that making any broad comments is tricky, but as a rule cichlids are intolerant of poor water quality, making them a bad choice for new aquaria. Many species are territorial and some are destructive, so keeping them in community and planted aquaria can be difficult. Most have specific water chemistry requirements that need to be met for successful maintenance. Rift Valley cichlids need hard and alkaline water, for example.

At the top end of the range are fishes that present the aquarist with significant problems and challenges. If you’re prepared to meet the needs of these fish, they can be extremely rewarding, which is why so many aquarists like to keep these fish. But these aren’t fish to buy on a whim. Good examples of “difficult” fish include discus, mormyrids (such as the elephantnose), stingrays, and many of the ambush predators such as needlefish and prehistoric monster fish.

Discus demand excellent water quality. Given their size and the fact they should be kept in a group, this translates into a demand for a large tank, generous filtration, and regular water changes. Discus also do best in soft and acidic water conditions, so aquarists keeping them in Southern England will need to find a way to soften and acidify the water supplied by the local water boards.

Mormyrids are widely sold as community tank oddballs but are actually rather difficult to keep. They are not fussy about water chemistry, but water quality is critical. They are also difficult to feed, and will not accept dried foods, at least not to begin with. Live foods, particularly worms and insect larvae, are essential. Most species are night-time feeders and won’t compete well with loaches and catfish. Finally, mormyrids are killed by many aquarium medications, making quarantining fish before adding to the mormyrid tank very important.
Stingrays, like mormyrids, are intolerant of poor water quality and most commercial medications. They are also very large and potentially dangerous, being equipped with a nasty sting.
Ambush predators are often very difficult to wean onto dead foods. Unless you have a supply of suitable live foods (such as earthworms and river shrimps) these fish are best avoided.

Aquarium maturity

Even if you choose hardy species, adding too many fish too soon to a new aquarium is asking for trouble. If you’re using fish to mature your aquarium, adding a couple of fish every ten days or so is the way to go. Danios and peppered catfish are both tried-and-trusted species for maturing aquaria this way, while mollies work very well in brackish and saltwater tanks. While maturing the aquarium, performing regular water changes is essential, because elevated levels of ammonia and nitrite will eventually kill even these hardy fishes. Both should be below 0.5 mg/l. If in doubt, doing a 25% water change every day for the first 2-3 weeks should keep your fish healthy during this difficult phase.

Once the tank is up and running, add new stock carefully and slowly, checking ammonia and nitrite periodically to make sure the biological filter is maturing nicely. Experienced aquarists can usually tell when fish are stressed by poor water quality, but newcomers to the hobby are advised to “go by the numbers” and use their test kits every day or two.

Tanks that are matured without using fish (so-called “fishless cycling”) or through the use of products such as SafeStart are different in some ways, but you still want to add hardy species to begin with. Measuring nitrite and ammonia after introducing the fish is still important, because even when the biological filter is kick-started using these methods there is still scope for problems. It takes anything up to three months for a biological filter to become truly stable.

Tank size

Fish require a certain amount of space to do well. Big fish obviously need a big aquarium, but very active small species should be given plenty of room as well.

Very small tanks (45 litres/10 gallons or less) are difficult to stock. Hyperactive species like danios are best avoided even though their small size would seem to make them ideal. Much better are small but relatively inactive species such as Siamese fighting fish, bumblebee gobies, neon tetras, Otocinclus catfish, kuhli loaches, or mated pairs of dwarf cichlids such as Apistogramma.

Small tanks in the 90-150 litre (20-30 gallon) range provide enough space for fish that like to swim about in fair sized schools. Small species of barb, danio, and tetra will generally do well in such aquaria. Corydoras, bristlenose plecs, and dwarf uspide-down catfish offer some useful options on the catfish front. Angelfish and gouramis can also work well, but these fish appreciate tanks with a certain amount of depth, so choosing tanks 45 cm (18 inches) in depth is a good idea when keeping these species.

Medium sized aquaria in the 180-250 litre (40-55 gallon) size bracket are ideal for communities of fairly large and active community fish, such as rainbowfish, mollies, swordtails, Congo tetras, and so on. A wide variety of catfish and loaches around the 10-15 cm (4-6 inch) size will also work well in such aquaria. Alternatively, these tanks can be used to house mated pairs of medium sized cichlids, such as convicts or jewel cichlids, or single specimens of robust predators such as bichirs and puffers. Tanks this size also provide an excellent choice for keeping collections of fancy goldfish that would tend not to do well in smaller aquaria.

Big tanks 700 litres (150 gallons) upwards are the ideal for “tankbusting” species such as arowanas, stingrays, oscars, clown loaches, and predatory catfish. Alternatively, such tanks can be used to house large schools of smaller fish, or else communities of species that require very good water quality, such as Tanganyikan cichlids.

Water chemistry

Water chemistry is the next thing to consider. Most tropical fish have a preferred range of water chemistry values in terms of pH and hardness. Broadly speaking, most of the tetras, barbs, South American cichlids, Corydoras, gouramis, and loaches prefer water that is neutral to slightly acidic with moderate to low hardness. Brackish water fish, livebearers, rainbowfish, Central American cichlids, and Rift Valley cichlids prefer water that is hard and alkaline.

Your retailer should be able to tell you the local water conditions, but failing that, a quick phone call to the water board will help. Choosing fishes that thrive in your local water conditions makes life much easier in several ways. For a start, you can perform large and frequent water changes without worrying about softening the water or adding hardening salts. Large water changes are one of the best ways to optimise water quality, especially when big and messy species are being kept. When fish are kept in conditions close to their optimal range, they are hardier and less likely to succumb to disease. Fish will also breed more willingly when kept in the correct water chemistry conditions.

Social behaviour: community tank or single species aquarium?

Fish vary in behaviour from territorial loners through to highly social schooling species. Barbs, tetras, rainbowfish, hatchetfish, danios, glass catfish, and Corydoras are all examples of schooling fish that must be kept in groups. Clown loaches are also schooling fish, despite their large adult size. When kept in too-small a group these fish are typically shy and nervous. Some species simply pine away, while others become frustrated and become positively antisocial, chasing or even nipping their tankmates.

Livebearers such as mollies and guppies are somewhat sociable in the wild but in aquaria the males tend to be aggressive towards one another. As a rule, these should be kept with one male to at least every two females; when kept in equal numbers the males invariably harass the females, sometimes to the point of death.

Some fishes are well known for being aggressive amongst themselves. Siamese fighting fish and wrestling halfbeaks represent one extreme, where the females largely ignore one another but the males are intensely aggressive and will fight to the death. Most cichlids are more or less territorial, and mated pairs especially will defend their “patch” against all comers.

A few fish sold in the trade are so territorial that they cannot be kept with other species under aquarium conditions. Puffers such as Tetraodon lineatus and catfish like Hemibagrus wyckii definitely fall into this bracket, being known to attack not just other fish but also things like heaters and filters as well as the arms of aquarists foolish enough to work inside the tank without first confining the fish! Other fish feed in such a particular manner that housing them with other species won’t work. The electric catfish Malapterurus spp. and the fin-eating characins such as Phago loricatus number among these.

Swimming level

Fish tend to prefer one of three levels in the water column: top, middle, and bottom. Surface-dwelling fishes include halfbeaks, hatchetfish, African butterflyfish, and danios. These fish will spend most of their time at the top of the tank, and often appreciate a mix of open water and a few patches of shade provided by floating plants.

Midwater species include most of the barbs and tetras in the trade as well as many different types of cichlid and gourami. As a rule the schooling fishes like barbs tend to enjoy open water, while the more solitary fish like gouramis skulk about among the plants.

The classic bottom-dwellers are loaches and catfish, but also numbered among these fishes are gobies, spiny eels, and most of the dwarf cichlids. These fish appreciate caves to hide in and rarely stray far into open water. Sand for digging into is always appreciated, and the aquarist will be rewarded for making an effort here by watching these fish behave in a more natural manner.

You want to choose fishes for all three levels of a community aquarium. A plan that works well is to choose one surface dwelling fish for every two midwater fish and every one bottom dweller, assuming all the fish are of roughly similar size. So mixing 6 zebra danios with 12 bleeding heart tetras and 6 bronze catfish will provide you with activity at all levels of the tank without any one part of the aquarium looking overcrowded.


Once you’ve established the kind of environment you can provide for your fish, and what sort of fish could work well in your aquarium, choosing fish becomes easier and more rewarding.

The value of a good aquarium encyclopaedia such as Baensch’s Aquarium Atlas cannot be overstated. Taking such a book with you to the aquarium shop lets you confirm details such as size and social behaviour with a high degree of confidence. If you’re shopping online, then Tropical Fish Finder will provide you will reliable and detailed information about fish species as well. Because this database is kept up-to-date as new fish are imported into the UK, it’s a good place to track down information on new and unusual species that aren’t featured in aquarium books.