FAQ: Filters for fish

Yes, warm water holds less oxygen while fish need more, so aeration helps on hot days. Bubbles and surface movement also increase evaporation slightly, which cools the water a little.

During the day, when CO2 is being added, strong aeration drives it off and reduces the benefit to plants, so the pump is often off then. At night, with CO2 switched off, aeration helps prevent oxygen shortage, so it is often run on a timer.

Water gets into the line when the pump switches off or stops and there is no check valve, or it has failed. Moisture can also condense inside. Fit or replace the check valve and keep the pump above the water level.

Yes, if the pump is powerful enough it can drive several devices through a manifold. Fit a valve on each line and adjust them so each device gets enough air.

Usually the stone is partly clogged or sits unevenly and the air takes the easiest path. Try turning it, weighting it to the bottom or cleaning it, and if that fails, replace it.

Most tropical fish need a steady temperature, usually 24–27 °C, which a room cannot hold without a heater. Coldwater fish such as goldfish do not normally need one unless the room gets too cold.

A common guideline is about 1 W per litre of water, a bit more in cool rooms or large tanks. Manufacturers state the tank volume a heater suits, and in very large tanks two smaller heaters are often more practical.

A heater is best placed where the water flow is strong, for example near the filter intake or outlet, so warm water spreads quickly through the tank. It must be submerged to the manufacturer's mark and not touch the substrate.

Many submersible heaters can be mounted at an angle or horizontally if the manufacturer allows it, which spreads heat more evenly. The thermostat part should not sit right above the heating element, and the heater must always be underwater.

Glass heaters are common and cheaper but can crack if dropped or switched on out of water. Titanium heaters or those with other tough housings resist knocks better, so they are often chosen for large or boisterous fish and turtles.

It is a heater fitted into an external filter's hose, so it is hidden from view and heats the water as it flows through. It suits display or large tanks, but the hose diameter must match the heater's fittings.

It is a cable laid under the substrate that gently warms the bottom and encourages water movement through it. It is sometimes used in planted tanks, but it usually supplements an ordinary heater rather than replacing it.

Set the desired temperature with the heater's dial or display, wait a day and check it with a separate thermometer. Heater dials are often inaccurate, so adjust according to the thermometer rather than the scale.

A glass heater that has been transported or stored in the cold must adjust to the water temperature, otherwise the sudden difference can crack it. The usual advice is to leave it submerged for 15–30 minutes before switching on.

No, a running heater heats up very fast in air and can crack, burn out or cause burns. Always switch it off before a water change that lowers the level and before lifting it out, and let it cool for at least a few minutes.

A plastic guard stops fish and turtles touching the hot surface and protects the glass from knocks. It is especially useful with turtles, large catfish and cichlids that can bump into or break the heater.

Warning signs are the temperature falling or rising regardless of the setting, the indicator light staying on constantly or never lighting, or a crack or water visible inside the glass. Check the thermometer daily, and if you suspect a fault, unplug the heater and replace it.

Switch the heater off at once, increase aeration and surface movement, and cool the water gradually by replacing a small portion with cooler, conditioned water. Avoid cooling it suddenly, and if the fish behave abnormally, contact a vet.

A heater's thermostat can be inaccurate or fail, and a thermometer lets you spot water that is too cold or too hot in time. It is cheap protection worth checking every day when feeding the fish.

There are glass submersible ones, adhesive strips on the outside of the glass, and digital ones with a probe. Strips are handy but approximate, while digital thermometers are more accurate and easy to read but need a battery.