FAQ: Filters for fish

Most hang-on filters have a control that reduces pump power or narrows the water path. Turn the flow down if the waterfall churns too much or bothers the fish, but remember the current still needs to reach the whole tank.

Most often the water level has dropped and the pump no longer reaches the water, or the intake and cartridge are clogged. Top up the tank, clean the intake tube and impeller, and with some models you need to fill the filter box by hand after it has stopped.

Manufacturers often advise regular cartridge replacement, but throwing one out also removes part of the useful bacteria. Many fishkeepers just rinse the cartridge in tank water and replace it when it falls apart, or add a sponge or ceramic media to the filter that stays put.

The air pump blows air into a tube inside the sponge, the rising bubbles pull water upward and fresh water is drawn in through the sponge. The sponge traps particles and bacteria settle in its pores, so it works both mechanically and biologically.

Manufacturers state the tank volume a sponge filter suits. The bigger the sponge, the more room for bacteria and the less often it needs cleaning, so a more heavily stocked tank is better served by a larger one or two filters.

Take out the sponge, gently squeeze it several times in a container of tank water until most of the muck comes out, and put it back. Do not use tap water or cleaners and do not scrub it spotless, so that bacteria remain.

The usual culprits are a clogged air stone, a kinked or cracked air line, a worn pump diaphragm or water too deep for a weak pump. Check the whole air path from pump to filter and replace the air stone if needed.

For small and medium tanks with light stocking, especially with shrimp or fry, a sponge filter is often enough. In large tanks or with messy fish it is usually used as an extra filter next to the main one.

A top filter sits on top of the tank, often as part of the lid: a pump lifts water, it trickles through the filter media and drips back. It is easy to reach for cleaning and aerates the water well, but takes space where lighting could go.

Switch the filter off immediately, open it and carefully remove the animal, which is often still alive. To prevent a repeat, fit a sponge or mesh over the intake and check that the filter lid and cartridge holder are closed tightly.

The usual order follows the water: first a coarse sponge or ceramic rings for large particles, then biological media, then a fine sponge or filter floss, with chemical media, if used, at the end. A specific filter's manual may say otherwise.

Ceramic rings are full of pores where nitrifying bacteria settle, making them one of the main biological filter media. They do not need replacing: an occasional rinse in tank water keeps the pores from clogging.

Plastic bio balls have a large outer surface but hardly any pores, so they suit wet-dry trickle filters best. Ceramic rings or sintered glass have many internal pores and hold more bacteria in a submerged filter.

Filter floss traps the finest particles and helps the water turn clear. It clogs quickly, so it is usually replaced often, and it is placed after the coarser sponges so it does not block straight away.

Ordinary cotton wool does not suit an aquarium filter, as it breaks down quickly and can come loose into the water. Pillow polyester stuffing is sometimes used, but it may be treated with chemicals, so floss made for aquariums is the safer choice.

A coarse sponge catches larger particles and clogs slowly, while a fine one traps small particles but clogs faster. That is why filters often combine them, coarse first and fine after. Both house bacteria.

Yes, aquarium sponges are often sold as sheets you can cut to fit a filter compartment. The sponge must fit snugly with no gaps, otherwise water flows around it and skips the filtration.

Universal sponges and cartridges often fit if they match the compartment's dimensions and are made for aquariums. Before buying, measure the old cartridge and check that the new one will not let water slip past the sides.

Activated carbon absorbs dissolved organic matter, smells, yellow tint and medication residues. It is not essential in every tank and stops working once saturated, so it is mostly used temporarily, for example after a treatment.

Carbon becomes saturated within a few weeks, depending on how dirty the tank is, and then stops working. Replacing it about monthly is the common advice if you use it all the time. The manufacturer states the exact interval on the pack.