The simple answer to this is ‘exactly the same as any other aquarium’, but this is not overly helpful. The three basic functions of an aquarium filter are to remove solid (visible) wastes, provide circulation and oxygenation, and provide an area for bacteria to grow (which remove harmful ammonia and nitrites). A good filter will also have some provision for chemical media such as carbon, which can help to keep water quality in top condition, although this type of provision is rare in small aquarium filters due to space restrictions.
The problem with packaged filters
Most small aquariums seen in retailers are designed with cost in mind, and as with most things which are low-cost, there are often drawbacks. The filters which come with many small aquariums are simply not capable of easy to manage and functional filtration. One of the biggest problems is a small single-piece of foam, which often makes up the entire filter media. Small foams measuring only a few centimetres will not trap a lot of visible waste, and they are often low quality, reducing the amount of surface area for bacteria to grow. Cleaning a small sponge is easy, but when it comes to replacing, the only option is to replace the whole sponge. By doing this, you remove all the useful bacteria that process wastes, and it will take time for bacteria to colonise the new sponge. After replacing a sponge, it is not uncommon to have water quality problems, which can affect the health of your fish and cause serious problems. If the sponge is big enough, it can be cut into two pieces so that you can replace just one half at a time, allowing the older half which still contains bacteria to colonise the new half.
Water testing
Small aquariums are more prone to water quality fluctuations, and because of a smaller volume, mistakes such as overfeeding (increasing the waste load) can have bigger repercussions. Combine this with a cheap, badly designed filter and we have a recipe for disaster – Ironically in the very aquariums that are marketed towards new and inexperienced fish keepers. The best way to avoid such problems is to carry out regular water testing, which every fish keeper, and particularly new fish keepers, should be doing. Testing for ammonia, nitrites, and nitrates on a regular basis – ideally every few days in a new tank and weekly for established tanks – will give you a good picture of how much water changing you should be doing, whether you are overfeeding, and if your filter is doing the job it should.
Choosing a new filter
If your small aquarium has a tiny, badly designed filter with just a single, small piece of foam, you may have no option but to fork out for a better filter. Expect to spend about £20-30 on a decent filter for a small (up to 50cm) tank and speak to your retailer about the best filters available. A new filter might be noticeably bigger in size so you will have to do some creative positioning, but the results will be worth it.
BiOrb and Reef-One filtration
The BiOrb, along with the BiUbe and LIFE aquariums made by Reef One, are a popular range of small aquariums that show how it is possible for aquarium manufacturers to produce decent filtration in small tanks. Due to the tanks popularity, it is worth looking at the filtration of these tanks, and how they have managed to address many of the problems with filtering small tanks. All of the aquariums made by Reef-One have a similar style of filtration so for simplicity we will refer to the whole range as ‘BiOrb’. The video opposite shows how a BiOrb filter works.
Separating biological media
In most small filters, the sponge does both jobs of trapping visible waste and housing bacteria. This causes problems when cleaning or replacing the sponge, during which bacteria is reduced or lost altogether. The BiOrb system separates the two processes by drawing water through a highly porous ceramic media, where the majority of bacteria settle, before it enters the sponge, which can then be used simply for trapping visible waste. This system allows the whole sponge to be easily replaced, without loosing essential filtration bacteria. The ceramic media does not need replacing and forms the aquarium substrate, although it can be quite sharp and some scavenging fish can damage their mouthparts whilst rummaging through it. This problem can be solved by adding a top layer of smooth gravel or stones – but make sure the gravel is not too fine or it will reduce water flow through the media.
Because the ceramic media is distributed around the base of the tank, rather than enclosed in a filtration unit, the BiOrb aquariums can house much more media than most small aquarium filters. If you gathered up all the filtration media in a BiOrb and tried to put it in a traditional internal filter, you would probably end up with something capable of filtering an aquarium several times the volume of a BiOrb. There is also more room for sponge, so more debris can be trapped, and the cartridges also contain some chemical media, so the filter can perform the full biological, mechanical, and chemical filtration processes.
Oxygen levelsIn small tanks, most filters situated near the waters surface will provide enough oxygen exchange. The BiOrb has a smaller surface area due to its shape, so it is easy to assume that the amount of oxygen that can enter the surface is reduced, which could cause problems for your fish. In fact, oxygen levels in BiOrbs are often higher than in traditional aquariums of a similar volume. This occurs because the BiOrb filter draws water in at the base and releases it at the surface via the use of a stream of air bubbles. Whilst the air bubbles help to oxygenate a bit, the biggest benefit is through the constant water movement through the whole tank. This movement ensures that water at the surface is continually ‘turned over’, creating a more constant rate of oxygen exchange.
The lessons to be learned with small aquariums is that it is worth investing a bit more to get a system with a decent filter included, or expect to purchase a separate filter with the new tank. A bargain tank is no use if it proves difficult to maintain and results in fish losses. If the filter looks small and flimsy, it probably needs replacing, and of course all of this is no use if you are not monitoring your water conditions.
