FAQ: Filters for fish

In hard water, limescale builds up on the impeller and housing. Plastic parts, without the filter media, can be soaked in a weak vinegar solution, scrubbed and rinsed very thoroughly in clean water before going back into the tank.

No, detergent and disinfectant residues are toxic to fish and kill the useful bacteria. Rinse filter media only in tank water, and if the filter has to be disinfected after a disease outbreak, rinse it thoroughly and restart it as you would a new one.

Without flowing water the bacteria in a filter start to suffocate within a few hours, so switch it off only briefly, for cleaning or feeding. If it has been off for more than a few hours, rinse it before restarting so foul water does not flush into the tank.

It is not necessary, but some fishkeepers turn the filter off briefly so the current does not carry sinking food into it and the fish can eat. Just do not forget to switch it back on; a filter's feed-pause function, where available, helps with this.

Better not, especially if it is the tank's only filter. Cleaning every sponge spotless loses a lot of bacteria and can cause an ammonia and nitrite spike. It is safer to clean in parts: half the media one time, the rest the next.

In a new filter the bacterial colony that breaks down ammonia and nitrite usually takes a few weeks to establish. Bacterial starter products or media from a mature filter can speed this up, and meanwhile fish should be added gradually.

Put an old sponge or some ceramic media into the new filter, or let both filters run together in the same tank for a while. That way the new filter takes over the bacterial colony and the old one can be removed after a few weeks.

Do not remove the old filter straight away: ideally run the new one alongside for a few weeks, or move some of the old media into the new filter. Feed lightly during this time and test for ammonia and nitrite.

Most often the sponges, intake or hoses are clogged, or dirt has built up on the impeller. Clean these spots and check that no air or snail has got into the pump, and if the flow does not recover the impeller may be worn.

Rattling is usually caused by a dirty or worn impeller, its shaft or rubber bushings, and gurgling by air inside the filter. Clean the impeller, check its parts, purge the air and make sure the filter is mounted firmly and not touching the glass.

Switching the filter off and on a few times or gently tilting the canister often pushes the air out. Also check that the intake is not too close to the surface, that the hose connections are tight and that the filter is not sucking in bubbles from an air stone.

Check that the socket has power, that the impeller is not jammed by dirt or a snail, and whether the pump has run dry. If the impeller is clean and spins freely but the filter still does not run, the motor may have failed.

Switch it off immediately, since a pump heats up quickly without water and the impeller and shaft can be damaged. Let it cool, check the parts for cracks, fill it with water and only then switch on, and keep an eye on the water level in future.

A rotten smell means a lot of organic muck has built up in the filter or it stood switched off for a long time without oxygen. Clean the filter in tank water, cut back on feeding and test for ammonia and nitrite.

Cleaning and restarting a filter often releases loosened particles into the tank, and if too many bacteria were lost, the water may turn milky from a bacterial bloom. It usually clears in a few days; next time clean more gently and in parts.

There may not be enough fine mechanical filtration, such as filter floss, or water is slipping around the sponge. The cloudiness may also come from bacteria or algae rather than particles, which a filter does not trap, so testing the water is worthwhile.

During treatment it is better to leave the filter alone, as it holds the bacteria that keep the water quality up. Some medications can harm these bacteria, so test ammonia and nitrite during and after treatment and follow the medication's instructions.

Filter floss traps a lot of fine dirt and clogs fast, so it is often changed every week or two, or when it turns noticeably dark and reduces the flow. As the bacteria live mostly in the other media, changing the floss hardly affects them.

If it is the only filter, better not to fit a new sponge straight away, since the whole bacterial colony goes out with the old one. Put the new one next to the old, and remove the old one only after a few weeks once the new sponge has been colonised.

A spare impeller, a seal for an external filter, suction cups, sponges and filter floss are useful to have. These parts wear out or go missing most often, and having them on hand means a filter fault does not cause long downtime.