FAQ: Filters for fish

Manufacturers state the tank volume a filter is made for. Choose by your aquarium's litres, and if the tank is heavily stocked or the fish are large, such as goldfish or cichlids, it is worth taking a filter rated for a somewhat bigger tank.

Filter sponges are rinsed regularly in aquarium water and replaced only when they wear out, fall apart or no longer spring back. Replace them a part at a time rather than all at once so you do not lose the useful bacteria.

The most common are internal filters, external (canister) filters, hang-on-back filters and sponge filters driven by an air pump. There are also undergravel, top-mounted and sump filters, which are more common in large or marine aquariums.

An internal filter is cheaper, easier to set up and suits small and medium tanks, but it takes up space inside. An external one holds more filter media, is quieter and needs cleaning less often, so it is the usual choice for larger aquariums.

Most aquariums do: a filter traps dirt, moves the water and, above all, houses the bacteria that break down ammonia, which is toxic to fish. Going without one only works in rare cases, such as a densely planted tank with very few animals.

Mechanical filtration traps visible particles, biological filtration happens in bacterial colonies that turn ammonia and nitrite into less harmful nitrate, and chemical filtration, such as activated carbon, removes dissolved substances, smells or discolouration.

It is an external filter: a sealed container usually placed in the cabinet under the tank. Water flows in through a hose, passes several layers of filter media and returns through another hose, so only the intake and outlet pipes remain in the aquarium.

It is a filter that hangs on the back wall of the tank: a tube draws water up and the filtered water falls back in like a waterfall. It takes no space inside, is easy to maintain and agitates the surface well, but needs a gap behind the tank and a cut-out in the lid.

A sponge filter is driven by an air pump, creates a gentle flow and does not suck in tiny animals, so it is popular for shrimp, fry, quarantine and breeding tanks. It is also cheap and reliable as an extra biological filter.

An undergravel filter pulls water down through the gravel, turning the whole substrate into a biological filter. It is simple but hard to clean, collects dirt and gets in the way of plant roots, so other filter types are more often chosen today.

A sump is a separate tank under the aquarium: water drains into it through an overflow, passes filter chambers and is pumped back. It adds water volume and hides the heater and other equipment, so it is used in large and marine aquariums.

Kits usually include an internal filter matched to that tank's volume, and it is normally enough for a moderate number of fish. If you plan many or large fish, feed heavily or want to clean less often, consider a stronger or additional filter.

Yes, two filters are common in large or heavily stocked tanks. It increases filtration capacity, and while you clean one filter the other keeps its bacterial colony going, so water quality fluctuates less.

It is how much water the filter pump can move per hour with no media in it. With loaded sponges and hoses the real flow is lower, so the figure on the box is a maximum, not a constant value.

A commonly cited guideline is a filter flow of 3–5 times the tank volume per hour, and more for goldfish or cichlids. It is only a guide: fish from calm waters, such as bettas, do not cope well with strong current.

Yes, if the current is so strong that fish cannot swim or rest calmly, plants bend over and the substrate gets stirred up. You can then turn down the flow control, aim the outlet at the glass or use a spray bar to spread the flow.

If the filter has a flow control, turn it down first. You can also aim the outlet at the glass or the surface, fit a spray bar with holes, break the flow with rocks or plants, and put a sponge over the intake.

A small tank suits a compact internal, hang-on or sponge filter with adjustable flow. The current must not be too strong, because in a small volume it is felt everywhere, and the filter should not take up half the space.

Large tanks usually get an external canister filter, two filters or a sump. These hold plenty of biological media and let you spread the flow across the whole tank so there are no dead spots.

Goldfish eat a lot and produce a lot of waste, so they need a filter with large biological capacity and stronger flow, often one rated for a bigger tank than yours. An external or powerful internal filter is the usual choice.