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  • Species: Boraras brigittae

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Stocking The Fluval Edge Aquarium

02 June 2010

The Fluval Edge aquarium is a 23-litre (5-Imperial gallon/6-US gallon) aquarium that includes a built-in filter and lighting system. Unlike most other aquaria, the Fluval Edge is designed to be viewed from all around, not just from the front. To make this possible the Fluval Edge includes both a glass lid and a compact module that keeps the filter and lights out of sight. The Fluval Edge is also unusual in being designed as a table-top aquarium rather than one that needs a special aquarium stand.

In this article we’ll look at a selection of fish species that can be kept successfully in the Fluval Edge provided good water quality and the right environmental conditions are maintained. Water chemistry may be a deciding factor in some situations, in which case it may be necessary to mix hard tap water with either rainwater or tap water that has been passed through a reverse-osmosis (RO) filter. Some aquarium shops sell RO water, and while not the cheapest option, for the volumes required to create the right conditions in the Fluval Edge this shouldn’t be too expensive. In Southern England for example, where hardness levels are typically around 15-20 degrees dH, a 50/50 mix of dechlorinated tap water with either rainwater or RO water generally produces acceptable conditions for most soft water fish.

Understanding the Fluval Edge

While the Fluval Edge aquarium is undeniably a clever piece of design, it’s important to recognise its limitations. Out of the box it lacks a heater, so anyone planning to keep tropical fish in a Fluval Edge will need to buy a small aquarium heater. Any 10-25 W heater should be sufficient for an aquarium as small as the Fluval Edge, but Fluval sell a special heater designed for the Fluval Edge that blends in with its overall design. Note however that the heater goes outside the module that contains the heater and lights, and not within it.

The very small volume of water also places some limits on what can be kept inside the Fluval Edge. Twenty-three litres is not a lot of water, little more than the capacity of a large bucket. Very few fish will be happy in a tank this small, and hardly any of the commonly traded community fish. Aquarists wanting to stock the Fluval Edge properly will need to choose their livestock very carefully, quite possibly using a mail order service to obtain species not on sale at their local aquarium shop. As noted above, water chemistry is a further limitation upon the range of viable species. If soft water fish are kept in hard water conditions they tend at best to lack their best colours, and at worst sickly and short-lived. Whether or not a heater is installed will further limit the range of possible options.

One problem with the Fluval Edge that isn’t immediately obvious is that only a small portion of the water’s surface area is in contact with the air. The top pane of glass is in direct contact with the surface of the water, and the only part of the water within a Fluval Edge that is in contact with the air is that part within the ‘collar’ that sits underneath the combination filter and lighting module. This has a couple of important consequences.

The first is that with limited area for gaseous exchange, the rate at which oxygen gets into the water will be less than that of an equivalent aquarium that has the entire surface of the water on contact with the air. To be fair on the Fluval Edge, the design of the filter means that water and air are quite well mixed within the filter itself, so provided the Fluval Edge is sensibly stocked, the amount of oxygen in the water shouldn’t be a limiting factor.

The second problem is that air-breathing fish won’t be able to come to the surface and gulp air as easily in a Fluval Edge as they can in a traditional aquarium, at least not if the aquarium is fully filled with water. For this reason, very small air-breathing fish species that might be kept in a very small aquarium, such as pygmy gouramis (Trichopsis pusilla) and dwarf corydoras (Corydoras habrosus) are probably not good choices for the Fluval Edge.

Hagen, the manufacturers of the Fluval Edge, recommend allowing 3 litres of water for every 1 cm of the fish’s length. If you take a small rasbora like Boraras brigittae, which reaches a length of 3.5 cm, then each Boraras brigittae will need 3.5 x 3 litres = 10.5 litres. Going by Hagen’s recommendation, the Fluval Edge would be suitable for just two of these tiny rasboras! Realistically, if you stick with fish as small as Boraras brigittae, you should be able to keep 5-6 specimens without problems, assuming of course that the aquarium is carefully maintained.

Although not a problem as such, one thing worth remembering about the Fluval Edge is that it needs to partially emptied before the aquarist cleans the inside of the tank. If that isn’t done, the aquarist’s hands will displace so much water that water is likely to rise up through the ‘collar’ and splash onto the table.


The built-in lighting used on the Fluval Edge incorporates a pair of small halogen spotlights like those used around the home. They aren’t designed for aquarium plants, and the type of light they produce isn’t ideal for plant growth. The small amount of space inside the Fluval Edge also means that aquarists won’t want to waste too much of that space for sand or gravel. Ideally, the substrate would be limited to just enough sand or gravel to cover the glass but no more. Less substrate means more water, and more water means healthier fish.

If those issues are taken together, then there are two obvious ways to plant the aquarium. One is to skip live plants altogether and use just plastic plants. This can work well, and the fish won’t mind either way. The other option is to use plants known as epiphytes. These are plants that grow attached to rocks or pieces of wood. They get all the nutrients they need from the water and don’t need any substrate at all. The standout species here are Java fern, Java moss, and the dwarf anubias (Anubias barteri var. ‘nana’) All three are very low maintenance plants, but they are tropical plants and will not do well at coldwater temperatures.

Algae control

The Fluval Edge is far too small for any algae-eating fish, even Otocinclus. Realistically, algae control will mostly consist of manually wiping down the glass with a sponge or scraper, but one or two small nerite snails might also be used. The snails feed on diatoms and green algae, and will not eat thread algae or blue-green algae. Nerite snails don’t eat much besides algae, and can easily starve to death; a Fluval Edge will be adequate for no more than two specimens 1 cm across.

Coldwater options

Goldfish are completely out of the equation here! While the Fluval Edge is lightyears ahead of any goldfish bowl, it’s still much too small for goldfish. Sure, a couple of juvenile goldfish 3-4 cm might be kept in a Fluval Edge for a while, but goldfish grow quickly, and within a few months these goldfish would need a new, much larger aquarium. Experts agree that adult goldfish shouldn’t be kept in aquaria smaller than 150 litres, and kept properly goldfish should be about 20 cm in length within a couple of years. So however tempting it might be, resist the urge to add goldfish to your Fluval Edge aquarium.

Hagen recommend the White Cloud Mountain minnow (Tanichthys albonubes) as a suitable species for the Fluval Edge, but in all seriousness, that’s a difficult piece of advice to agree with. For a start, you’d need 5-6 specimens for them to be happy. Even allowing for Hagen’s rather conservative estimate on stocking density, six 4-cm long fish will be well above what the Fluval Edge is designed to hold. A better coldwater choice would be the dwarf mosquitofish (Heterandria formosa). This small livebearer will be happy down to about 18 degrees C, so should do well in an unheated Fluval Edge positioned in a warm, centrally-heated room. A trio (one male, two females) would be happy in this aquarium provided it was well planted and the water was at least moderately hard (10+ degrees dH, pH 7-8).

Cherry shrimps or bee shrimps would make colourful additions to a coldwater Fluval Edge. Again, they are perfectly happy at the temperature of a centrally-heated room. Cherry shrimps are not fussy about water chemistry, but bee shrimps prefer water that is slightly soft and acidic (5-10 degrees dH, pH 6.5-7.5). These shrimps are primarily algae-eaters, but they also eat leftover fish food, so would be useful companions for dwarf mosquitofish. Three or four cherry or bee shrimps could be kept with a trio of dwarf mosquitofish. It’s best to keep a group of one particular shrimp species rather than a couple of both because they are sociable animals that become shy if kept in insufficient numbers.

Tropical options: colourful schooling fish

One of the most popular species for the Fluval Edge aquarium is the galaxy rasbora (Danio margaritatus). This little fish only gets to about 1.5 cm long but is very colourful and lively, and while it looks best given more space, a Fluval Edge is just about big enough for a school of 6 specimens. They are not fussy about water chemistry but extremes should be avoided; aim for 10 degrees dH, pH 6.5-7.5. The water temperature needs to be around 24-25 degrees C. Like a lot of schooling fish galaxy rasboras appreciate a mix of open swimming space together with some plants for shade and shelter.

Various tiny Boraras species could be maintained in the Fluval Edge aquarium instead of galaxy rasboras. The most widely traded species is Boraras brigittae, sometimes called the ‘chilli rasbora’ or the ‘dwarf redfin rasbora’. This fish gets to about 3 cm long and a group of 5-6 specimens could be maintained in a Fluval Edge. Because they’re bright red, these fish would certainly be very eye-catching. Less often seen species that could be kept in the same way include Boraras maculatus, Boraras micros and Boraras urophthalmoides. All Boraras species need soft, slightly acidic water to do well; aim for 5-10 degrees dH, pH 6-7.

Tropical options: for advanced aquarists

Danionella translucida is oddball schooling fish and reckoned to be among the world’s smallest freshwater fish. Adults are barely 1 cm long, so a school of 6 specimens would work well in a Fluval Edge. This species is a kind of danio, but so small that it makes a poor community fish (in fact it usually ends up as live food when added to a mixed species aquarium). But it does get along well with small shrimps such as bee shrimps. Danionella translucida isn’t too fussy about water chemistry provided extremes are avoided.

The paradox fish (Indostomus paradoxus) is a tiny pipefish-like fish that feeds exclusively on tiny live foods. Although not delicate, it is difficult to maintain unless suitably small live foods are regularly provided. These include things like daphnia, microworms, brine shrimp nauplii and cyclops. They get along well with small shrimps but cannot really be kept with other types of fish. 3-4 specimens would do well in a densely planted Fluval Edge. Water chemistry isn’t critical, but aim for as with most Southeast Asian fish, relatively soft, slightly acidic to neutral water is best; 5-15 degrees dH, pH 6-7.5.

Desert gobies (Chlamydogobius eremius) are Australian gobies that inhabit pools where water chemistry and temperature change rapidly. Like most gobies they prefer live and wet-frozen foods, but they are otherwise not difficult to keep. Males are larger and more colourful than the females, and considerably more aggressive. A trio consisting of one male and two females will work in Fluval Edge provide it is nicely planted and includes several hiding places such as caves or snail shells. Desert gobies must have hard water to do well; aim for 15-25 degrees dH, pH 7-8. The addition of marine salt mix at 2-3 grammes per litre is beneficial and will not harm the epiphytic plants mentioned above. Desert gobies are not fussy about temperature and can survive in unheated tanks, but they do better in at tropical temperatures.