FAQ: Filters for fish

Yes, activated carbon absorbs many medications and weakens them, so it is usually taken out during treatment. The medication's instructions say whether this is needed, and if in doubt it is worth asking a vet.

Rinsing only removes dust and dirt from the surface and does not restore carbon's ability to absorb. Saturated carbon must be replaced, and rinsing is only worth doing before first use so it does not cloud the water.

Zeolite is a mineral that absorbs ammonia, so it is used when ammonia has to be cut quickly, for example during transport or in overstocked tanks. It becomes saturated and is no substitute for a biological filter, and it works less well in salt water.

Peat releases humic substances that slightly soften and acidify the water and tint it amber. It is used in tanks with fish from soft, acidic water. Use peat made for aquariums, without fertiliser or additives, and check the pH regularly.

It is granules or pads that bind phosphate from the water, so it is sometimes used against algae. It does not fix the cause, such as overfeeding or infrequent water changes, and must be replaced once saturated.

These are special resins that remove nitrate from the water in exchange for other ions. They can help when tap water is high in nitrate, but they only supplement regular water changes and moderate feeding, not replace them.

It is porous glass cylinders or balls with a huge number of micropores inside for bacteria. It is considered one of the most effective biological media and, like ceramic, is not replaced but only rinsed occasionally in tank water.

Usually not: ceramic rings last for years as long as their pores do not clog. If they have become heavily coated with sludge, rinse them in tank water, and if you do replace them, change only part at a time, such as a third.

The more fish and food, the more biological media is needed for the bacteria to keep up with the ammonia. In practice most of the filter's volume goes to biological media, leaving only as much as necessary for mechanical and chemical media.

Mesh bags are handy for loose media such as ceramic, carbon granules or zeolite, since they are easier to remove and rinse. The bag should not be overfilled, so that water flows through all the media rather than around it.

Most sponges and ceramics do not, but peat acidifies the water, crushed coral or limestone chips harden it and raise the pH, and resins change its ion balance. When using such media, test the water regularly.

Crushed coral slowly dissolves, raising water hardness and pH, so it is used in tanks with fish from hard, alkaline water, such as Malawi cichlids. It is not suitable for soft-water fish.

Store new media in a dry place in sealed packaging, especially activated carbon and zeolite, which also absorb substances from the air. Used biological media should not be kept dry for long, as the bacteria die.

New carbon contains fine dust that darkens or clouds the water once it gets in. Rinse the carbon well under running water before putting it in the filter, until the water runs clear.

Sponges, ceramics and sintered glass suit both. Some chemical media, though, are made for one water type only, for example zeolite works less well in salt water, so check what the manufacturer states.

It depends on filter size and stocking: internal filters are often cleaned every 2–4 weeks, external ones every 1–3 months. The best sign is a noticeably weaker flow rather than the calendar, since cleaning too often disturbs the bacteria.

Chlorine or chloramine in tap water and a sudden temperature difference can kill part of the useful bacteria in the filter media. Rinse sponges and ceramics in tank water drained into a separate container during a water change.

Many people do, because tank water is then at hand for rinsing the sponges. It is better, though, not to combine a large water change, gravel vacuuming and a full filter clean all at once, as that disturbs a lot of bacteria in one go.

Switch the filter off, close the taps, disconnect it and take the canister to a sink or bucket. Rinse the sponges and other media in tank water, clean the impeller and check the seal, then reassemble, fill with water and restart.

Remove the impeller from the pump housing, clean off the slime and deposits with a soft brush or an old toothbrush, and clean its chamber and shaft too. A clean impeller reduces noise and restores flow, and a worn or cracked one is worth replacing.