Choosing and Using an Aquarium Filter

The importance of proper filtration cannot be overstated. Get the water quality right, and everything else in fishkeeping is easy. Fail to keep the water clean, and your fish will be sickly and probably die. Given that all filters work in the same basic way, it might be surprise to see just how many different designs and models there on the market. Each has its pros and cons, and choosing the right one is one of the key steps to setting up a new aquarium, so it is worth spending some time learning about filters and understanding how the different designs work.

The basics

All filters work in the same basic way. A pump of some sort pulls water through one or more types of filter medium, where the water is cleaned and perhaps chemically adjusted in some way as well. A very simple filter like an air-powered sponge filter performs biological and mechanical filtration, specifically bacteria coating the sponge convert toxic ammonia into less toxic nitrite and then relatively harmless nitrate (biological filtration), and the pores in the sponge collect particles of solid waste in the water such as silt (mechanical filtration). More complex filters add chemical filtration to the mix. Peat granulate might be added to a canister filter so that the water is softened and acidified, making it more suitable for blackwater fish like rasboras and discus.


One of the main differences between filters is the rate at which they push water through the media. This is known as the turnover, and is usually quoted in terms of LPH (litres per hour) or GPH (gallons per hour). For example, a medium sized external canister filter might have be rated at a turnover of 700 LPH. What does this mean in real terms? It means that the water in a 175 litre aquarium would pass through the water 4 times per hour. This of course begs the question of how often should the water be passed through a filter for adequate filtration to be performed. The answer to this depends upon the type of fish being kept. Small, community tropicals like guppies and neons will be well served with a filter turning the water over 4 times per hour. Larger, messier fish like goldfish and cichlids will need a higher rate of turnover, around 6 times per hour being the minimum. Fast-water tropicals like giant danios, hillstream trout, loaches, and many suckermouth catfish will also appreciate similarly high levels of water movement. A fish-only marine aquarium containing hardy species like damsels will require a turnover of no less than 6 times per hour, and in reef-tanks a turnover of up to 10 times per hour is recommended.

A complicating factor is that the quoted turnover rate on the packaging is unlikely to be achieved in reality. Firstly, once the filter media is installed, the turnover will drop, and it will continue to decline as the media becomes clogged up with solid waste. After a few weeks of use, the filter might only achieve around half of its quoted turnover rate. Secondly, if the filter is located under the aquarium, the pump will have to work harder to move water against gravity, "head pressure", with the result that the turnover rate will drop even more.

Determining 'real world' turnover rates is potentially possible once you have the filter installed, by placing the outlet in a bucket at the level of the aquarium and seeing how long it takes to fill that bucket and then multiplying the result appropriately to see how much water the filter would move in one hour. But this is obviously not much help when shopping! As a general rule, filters at the level of the tank (like internal filters or filters that hang on the back of the aquarium) don't suffer from the head pressure problem and the only thing reducing the flow of water will be the filter media and the rate at which the media clogs up over time. In an aquarium with relatively clean fishes, like a selection of small community tropicals or a breeding pair of dwarf cichlids, the quoted turnover rate on the filter won't be far off the real world turnover, and all you probably need allow is a margin of 10-20%. In other words, a 70 litre aquarium of this type would require a turnover of about 4 x 70 = 280 LPH, and allowing a margin for dirty filter media, you could round that up and choose an internal filter with a turnover of around 300-330 LPH.

Now, a bigger tank containing larger fish like plecs, oscars, or arowana would require significantly more filtration. External filters are better value in these situations because they tend to provide more turnover per £, but on the downside the filter will likely need to be positioned in the cabinet under the tank, meaning that head pressure becomes a factor. You'll need a turnover of around 6 times per hour just to keep pace with the mess these fish create. So for a 400 litre tank, you need a filter with a turnover of around 2400 LPH. To account for clogged-up media and the head pressure problem, factor in a margin of 25-50% to be on the safe side, so round that up to 3000-3600 LPH. To get this much filtration, you may need to use more than one canister filter, but having multiple filters actually offers a safety net of sorts: should one filter fail, the other will pick up some of the slack, giving you time to fix or replace the broken filter.

Internal or external?

Although there are lots of other types of filter on the market, the two most popular designs in Britain are internal power filters and external canister filters. Each has its pros and cons. Internal power filters, like the 'Eheim Aquaball' series, consist of an electrical pump known as a powerhead that draws water through one or more filter medium chambers. The whole thing sits inside the tank, usually attached to the glass via rubber suckers or a plastic clip. In most cases, an airline tube can be attached to the powerhead to add air to the outgoing flow of water; while this decreases the turnover slightly the increased turbulence improves the rate at which oxygen can diffuse into the water. Of course, increased turbulence also drives off carbon dioxide, which may be a bad thing in a planted aquarium where carbon dioxide fertilisation is being employed. The key advantages to the internal power filter are that it is operating at the same level as the aquarium so there is no head pressure for the pump to work against and that it is usually very easy to remove and clean when required. The main disadvantages of internal filters are that they tend to be expensive in terms of turnover per £ and that being installed inside the tank they are unsightly unless carefully hidden behind plants or rockwork. Internal power filters also tend to be rather small, so while adequate for standard community tropicals, they are less useful in tanks with big, messy fish.

External canister filters such as the 'Fluval 204' use a large powerhead to pull water through a series of baskets containing filter media within a watertight chamber. Water is drawn in through one pipe, the inlet, and pushed out through another, the outlet. The outlet may consist of a simple spout, a spray bar, or a venturi device that mixes air and water at high pressure. On the plus side, external canister filters are generally very good value in terms of turnover per £, and for this reason have become very popular for use on large aquaria where lots of filtration and a strong water current is required. Because they can be hidden away inside a cabinet under the tank, they don't interfere with the layout of the aquarium, making them a good choice for tanks where aesthetics are important. The chief downsides to external canister filters are that they need to work against head pressure and that taking them apart for cleaning is a bit of a chore. To be fair, external canister filters are easier than ever to maintain, but even so they are comparatively fiddly compared with internal filters, especially with regard to disassembly (making sure no water is spilled when the pipes are disconnected) and priming (starting the water flow again once the filter is re-assembled).

Built-in filters

Many aquaria come with built-in filters of various types. Some are simply regular internal power filters slotted into a hidden compartment. Others, like the popular 'Rio' aquaria, have a filter built into one corner that consists of a series of sponges in a large chamber into which water enters through a sluice. Water is pumped out by a relatively small powerhead because it doesn't have to work against head pressure, and because the filter chamber is essentially open to the air, the biological filtration potential is very high thanks to the abundance of oxygen and size of the sponges. In terms of advantages and disadvantages, such filters are comparable to large internal power filters. In many cases, such filters are best suited to freshwater community tank situations, and if the aquarium is used to hold large freshwater fish or marines, then the addition of a second filter, ideally an external canister filter, is usually essential.

Undergravel filters

The standard filter for many decades was the undergravel filter, though such filters are relatively rarely seen nowadays. Undergravel filters consist of a reasonably deep gravel substrate (typically around 6-8 cm) resting on top of a large plate with holes in it. Water is pulled through the gravel into the plate and up a tube in one corner of the tank by a powerhead or airstone. The main advantage of an undergravel filter is that it performs a lot of biological filtration for very little cost, since the media, the gravel, is already in the aquarium anyway and the powerhead isn't working against head pressure so doesn't need to be terribly strong. Hence, even an airstone or two can do the trick. In marine and hardwater cichlid tanks, a substrate of crushed coral topped off with coral sand works works particularly well, not only providing good biological filtration but also buffering the water against changes in pH and hardness.

The major disadvantages of undergravel filters are that they draw solid waste into the gravel, and so the gravel needs to be regularly stirred and the 'mulm' siphoned off. Once a year it is usually necessary to take an undergravel filter apart to clean underneath the filter plate, but this is obviously a major hassle because the tank effectively has to be emptied and taken apart and all the fish placed into buckets during the operation. Because the gravel is the filter bed, anything that blocks off substantial parts of the gravel from the water flow, such as large rocks, reduces the filtration capacity of the system, and this limits the options in terms of aquascaping. Similarly, sediments that prevent water flow altogether, such as sand, cannot be used at all. Finally, undergravel filters are also problematical with plants. Most species of plant do not like having oxygenated water flowing past their roots because it interferes with mineral uptake. So while floating plants or epiphytic plants like Java ferns work fine with undergravel filters, with most other plants undergravel filters are best avoided.

The upshot of all this is that despite their effectiveness and good value for money, undergravel filters have fallen out of fashion in recent years, with most aquarists opting for systems that are more versatile and easier to maintain even if somewhat more expensive to buy.

Media options

The range of filter media on sale can be bewildering. The three most important media are filter wool (or floss), sponges, and ceramic media. Filter wool is primarily used for trapping small particles and though it can be cleaned and reused to some extend, eventually it gets so dirty and matted that this doesn't really work any more and it needs to be replaced. Sponges can be used for both mechanical and biological filtration. Typically, coarse sponges are used for mechanical filtration and finer sponges for biological filtration but some filters may only have a single large sponge that does both. Either way, sponges need to be cleaned periodically. If the sponge is used purely for mechanical filtration, it can be rinsed under a tap, but otherwise the sponge must be cleaned in a bucket of aquarium water so that the filter bacteria are unharmed. Modern sponges typically last for several years, though eventually they may become irretrievably clogged and need replacing.

Ceramic filter media are also available in coarse and fine grades. A coarse ceramic medium, such as 'Ehfimech' or 'Fluval Pre-Filter' is designed primarily to work as a mechanical filter medium, whereas a finer ceramic medium such as 'Siporax' or 'Ehfisubstrat' will support a much denser population of bacteria and so works best as a biological filter medium. Because ceramic media are chemically inert and very durable, they will last for many years. Good quality ceramic media are a very sound investment, not only because they last for a long time but also because as biological media especially they tend to be extremely effective.

Chemical media

Besides the basic mechanical and biological filtration media, additional chemical filtration media are also available. One of the most useful is zeolite, a mineral that absorbs ammonia. Used instead of biological filtration, zeolite is ideal for situations where there isn't time to mature an aquarium in the normal way, for example when a breeding tank or quarantine tank needs to be set up for immediate use. Zeolite also works well below pH 6.0, whereas biological filters do not, so is the filter of choice for aquarists keeping species that inhabit very acidic water conditions. The catch is that zeolite only works up to a point, and once saturated, performs no filtration at all. Careful monitoring of the ammonia levels in the aquarium is important, and the zeolite needs to be replaced before it becomes saturated with ammonia. Zeolite can be "recharged" in brine, and in small tanks at least can be a very cost effective form of filtration.

Carbon is another widely sold chemical medium, but its uses are much more limited. Carbon removes dissolved organic chemicals, including medications. In the past, when aquarists believed 'old water' was better than 'new water', anything that prevented the need for water changes was deemed to be useful. Carbon did this, by preventing the water becoming yellow with dissolved organics. Nowadays frequent water changes and, in brackish and marine tanks the use of protein skimmers, makes carbon largely redundant. Also, because carbon needs to be removed before medications are used, adding carbon to filters that are difficult to maintain anyway, like canister filters, can be an annoyance.

Peat granulate is a chemical medium for use when fish from soft, acidic waters are being kept. Peat softens and acidifies water. In very hard water the effect is usually slight, but in softer water peat can change the pH very rapidly, so it is important to combine the use of peat with regular testing of the pH and hardness. Peat also darkens the water, lending it a tea-coloured stain that highlights the colours of many fish, particularly killifish, tetras, and dwarf cichlids.

Making your choice

Choosing a filter is one of the most important decisions you will make when setting up a new aquarium. It is always important to investigate your options and balance your demands against your budget. Scrimping on the filter is rarely sensible because without adequate filtration, nothing in the tank is likely to do well. The simple advice "buy the best you can afford" is particularly true here, because if it is working properly, and appropriate to the size of the job you have set it, the filter is something you need never think about except when it needs to be cleaned.

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