Which filter for your aquarium?
The different aquarium filters
There are 5 main types of filters:
Internal power filters
Economical and very popular, the internal power filter may be used for tanks up to about 150 liters/40 US gallons for the largest types.
Power filters work by drawing water from the aquarium, through the filtration medium before returning it to the tank. Power filters can combine both mechanical and biological filtration and are mounted to one of the sides of the tank with suction cups. Some models offer an adjustable flow rate and a “venturi” system to ‘aerate the water’ thanks to small bubbles. Power filters are usually very easy to maintain. For goldfish, avoid the ‘mini-filters’ as they have been designed for small tanks or fish and therefore do not have the capacity to clean the waste produced by goldfish. In conclusion, provided that one does not choose a model that is too small, the internal power filter is easy to maintain and well suited to small to medium sized tanks. Good brands are: Fluval, Eheim, JBL, Superfish (if you have a very limited budget), All Ponds Solutions, API/Rena.
Hang On Back Filters (waterfall filter)
This filter hangs over the back or side of your aquarium and sucks water up through a siphon tube.
A water pump, located in the last compartment of the filter, sucks in and circulates the water through the various filtration media (e.g. sponge or wool, ceramics bio media, cartridge, etc.) to discharge it purified to the surface of the tank.
The biological filtration is better than that of the internal power filter because the volume of filtration media is bigger. Hang on back filters are efficient, economical, easy to maintain and well adapted to medium volumes. However, these filters are a little bulky.
Canister Filter (external filters)
Canister filters are kept outside of the aquarium. They used to be expensive but there are now many affordable and reliable models available.
An effective option for medium to large volumes (from about 100 liters / 25 US gallons for the most compact models) because external filters contain a very large amount of filter media. External filters are very versatile: the choice of filtration media is very wide – sponge, ceramic bio media, activated carbon…
The two pipes (inlet and outlet) of the filter are easy to hide, behind plants or decoration for example. It is necessary to house the equipment (usually in the cabinet underneath the tank) and to check from time to time that the pipes are watertight. As it is outside the tank, the canister filter can be cleaned without disturbing the fish. Good brands are: Fluval, Eheim, JBL, Superfish (if you have a very limited budget), All Ponds Solutions, API/Rena.
The box filter (sometimes called corner filter) used to be very popular. It is nowadays supplanted by power filters.
Corner filters are a super cheap option, providing you already have an air pump. Otherwise, in most cases, a power filter is a much better option. The bubbles produced by the air pump (placed outside the tank) circulates the water through the filtration media and at the same time ensures efficient aeration of the aquarium.
Biological filtration is not too bad, providing the box filter is large enough. However, mechanical filtration (suction of larger particles) is poor. And box filters are ugly. These filters are often used in breeding tanks however, where power filters would create too much movement for the fry and could suck the eggs or tiny fish. For goldfish, it can be a good choice for the quarantine tank (easy bacterial seeding) or as an emergency filter (if your main filter stops working).
Sponge/air driven filter
A cheap and old-style filter that works very much like the box filter.
Water is pulled through the sponge via an air pump. Like corner filters, biological filtration is decent but mechanical filtration is poor. Best for breeding tanks, fry (to avoid injuries to the baby fish), hospital tanks or aquariums with invertebrates.
Another blast from the past. Used to be hugely popular but is now supplanted by power filters too.
This cheap and discreet filter is only suitable for small volumes and requires the prior installation of perforated plastic plates placed slightly sloping under the aquarium floor (recommended gravel thickness above: 75-80mm; gravel diameter: 3-5mm). The water is then sucked through the gravel and ejected back into the tank thanks to an air column (vertical tube) generated by an air pump (aerator) and/or a water pump depending on the type. It is the gravel that actually acts as a filtration material here: particles and organic waste are attracted under the substrate (mechanical filtration) and then degraded by the action of bacteria naturally present in the gravel (biological action).
However, these filters have many disadvantages: it must be placed first when the aquarium is installed and cannot be changed without removing everything. The biological efficiency is very good but the mechanical efficiency is poor. Chemical filtration is not possible with this type of filter. Maintenance must be very regular, otherwise the filter may slow down or even clog up. Finally, it is not well suited to plant growth because its action disturbs the roots and any plant substrate is prohibited (note: it is always possible to have plants that do not require rooting, such as Anubia or Java fern).
Filter flow rate and filter maintenance:
Whichever filter chosen, the hourly filtration rate must be at least twice the volume of water in the aquarium. More and more filters are now equipped with variable flow rates, which allows you to adapt the flow rate to your needs.
When the flow rate drops significantly, rinse the filtration media with tank water, but never with tap water because the filtration media contains essential purifying bacteria that are very sensitive to chlorine.
If your aquarium filter is equipped with just one filter medium (a single sponge for example), care should be taken before changing the sponge to avoid sacrificing the entire bacterial colony at once: simply cut the sponge block in half. Only change one half at a time and the second half two weeks later. The old sponge will then have the time to colonize the new one and enough bacteria will be preserved!
Important: the aquarium filter must run 24 hours a day, 7 days a week.
Dennerle, Rena API, Eheim, JBL, Tetra, Fluval for example.
Efficient biological filtration:
A large amount of filtration media means a large surface for the beneficial bacteria and a more effective biological filtration. This pleads in favor of a filter of a size suitable for the goldfish tank because they produce lots of waste.