FAQ: Filters for fish

Not always: if the filter moves the surface well, there is often enough oxygen. An air pump helps in hot summers, in heavily stocked tanks, during treatments, at night in densely planted tanks and for running sponge filters.

Manufacturers state the tank volume or depth an air pump suits, plus its air output in litres per hour. The deeper the water and the more air stones or filters you connect, the more powerful the pump needs to be.

Stand the pump on something soft, such as a mat or sponge, so it does not vibrate against the furniture, and do not hang it on the glass. If it keeps getting louder, the diaphragm may be worn, and models with several vibration-damping feet are usually quietest.

A check valve stops tank water from siphoning back down the air line into the pump when it switches off, for example in a power cut. It is especially needed when the pump sits below the water line, and it is fitted with the arrow pointing in the direction of airflow.

It is safest to place the pump above the water level so water cannot flow back if the power goes out. If it has to go lower, fit a check valve or loop the air line above the tank's rim.

An air stone splits the pump's air into fine bubbles. The bubbles themselves dissolve little oxygen; the key effect is that the rising stream moves the surface, where gas exchange happens. Air stones come in many shapes and sizes, including decorative ones.

Over time an air stone's pores clog with deposits and algae, the bubbles get bigger and the pump has to work harder. Many people replace air stones every few months, or when the bubbling noticeably weakens or becomes uneven.

Use an air manifold with valves that splits one line into several, and balance the airflow to each line with the valves. Remember each extra air stone reduces the flow, so the pump must be powerful enough.

Use tubing made for aquariums, usually 4–6 mm inner diameter, that fits the pump and air stone connectors. Silicone tubing is softer and flexible, while stiffer PVC tubing kinks less but can harden over time.

Yes, aquarium air pumps are built for continuous running and can operate day and night. In densely planted tanks some people run it only at night, when plants stop producing oxygen, and switch it off by day to avoid losing CO2.

Strong aeration drives carbon dioxide out of the water, which plants need, especially if you add it. In densely planted tanks aeration is often used only at night, or only the surface movement the filter creates is kept.

Bubbles do not bother many fish, and some like swimming in the stream. For bettas, gouramis and other still-water fish, though, a strong bubble stream can be stressful, so aeration for them should be gentle or tucked in a corner.

Most often the diaphragm is worn or torn, the air stone is clogged or the line is kinked. Check whether air comes out of the pump with the line disconnected: if not, the diaphragm can often be replaced with a repair kit, and if so, look for a blockage in the line.

Many air pumps have an output control, and if not, airflow can be reduced with a valve in the line. It is better to bleed off surplus air through a separate line than to close the valve fully, so the diaphragm does not work against too much pressure.

It is a small air pump that runs on batteries or a rechargeable cell. It is useful when transporting fish, during power cuts or for fishing bait, when a mains pump cannot be used. Some models switch on automatically when the power fails.

Do not feed the fish, since food and digestion raise their oxygen needs, and avoid opening the tank unnecessarily. A battery air pump helps, or you can scoop and pour water back now and then to keep the surface moving.

Fish gather at the surface gulping air, breathe fast and are sluggish, and this is often worse in the morning. Increase surface movement, turn on aeration, check the temperature and filter, and if things do not improve, contact a vet.

Ornaments moved by an air stream, such as opening shells or bubbling chests, connect to an ordinary air pump and add water movement. Choose ornaments without sharp edges, and make sure fish cannot get trapped inside.

It is a long air stone or perforated tubing laid along the bottom or back wall, from which a wall of bubbles rises. It is decorative but also moves the water well along the whole length of the tank.

Some air stones can be revived by soaking them in hot water or a weak vinegar solution and rinsing well, but replacing one is often cheaper and simpler. Rinse the stone thoroughly in clean water before returning it to the tank.