FAQ: Filters for fish

It is a separate device with a probe in the water that switches the heater on and off according to an accurate temperature. It adds protection against overheating if the heater's own thermostat fails, so it is used with valuable or sensitive fish.

In a cold room the heater has to work harder, so an underpowered model may not hold the temperature. If the tank stands somewhere cool, near a window or the house is kept cool in winter, choose a somewhat stronger heater or a lid that cuts heat loss.

Two smaller heaters warm the water more evenly at both ends of the tank, and if one fails the other stops the water cooling quickly. Also, if one sticks on, a smaller heater overheats the water more slowly.

Goldfish are coldwater fish and usually do well at room temperature, roughly 18–22 °C. They rarely need a heater, but sudden temperature swings, such as next to a radiator or in a sunny spot, should be avoided.

Bettas are tropical fish and do best in water of roughly 25–28 °C. Even in a small tank they usually need a small heater with a thermostat, since room temperature is often too low and fluctuates too much for them.

Heaters without a thermostat heat continuously, so in a small tank the water can overheat, especially on a warm day. A small heater with a thermostat or a preset temperature is safer, checked with a thermometer.

Turn off the heater, open the lid and point a fan at the water surface so evaporation cools it, shorten the lighting period and keep the tank out of direct sun. Increase aeration, since warm water holds less oxygen.

Aquarium fans clip to the rim and blow across the surface, lowering the temperature by a few degrees through evaporation, so you will need to top up more often. A chiller is a refrigeration unit connected to the filter that holds the temperature precisely, but it is expensive and used for sensitive tanks.

Ice or frozen water bottles cool unevenly and abruptly, which can stress fish, and melting tap water is not conditioned. If you must, use sealed bottles of water, add one at a time and watch the thermometer.

It means the heater is heating non-stop: the water has not reached the set temperature yet, the heater is too weak, the room is very cold or the thermostat has failed. Check with a thermometer that the temperature is rising, and if it goes above the setting, unplug the heater.

It is a pump with no filter media that only moves water and creates current. It is used in large tanks to avoid dead spots, in tanks for river fish and in marine aquariums where strong circulation is needed.

Strong flow is enjoyed by fish from rivers and fast streams, such as some barbs, loaches, catfish, gobies and certain riverine cichlids. Even they need calmer spots to rest, so it is worth aiming the current at one part of the tank.

It is the pump in the sump that sends filtered water back to the aquarium. Its output must match what the overflow can handle, so the sump neither drains too fast nor overfills the tank, and it is sized taking the lift height into account.

It is the maximum height to which a pump can lift water. The higher the water is lifted, the lower the flow becomes, so when choosing a pump for a sump or fountain look at the flow-versus-height table, not just the maximum output.

A UV steriliser passes water past a lamp, and the ultraviolet light kills free-floating algae, bacteria and some parasites. It helps against green water but does not replace the filter or cure fish that are already sick.

A UV lamp loses effectiveness over time even if it still glows, so it is usually replaced about once a year or after the operating hours the manufacturer states. The quartz sleeve also needs cleaning, as deposits block the light.

No, because most of the useful bacteria live attached to filter media and surfaces, not free in the water. UV only acts on water flowing past the lamp, so it essentially does not interfere with biological filtration.

UV light breaks down some medications and water additives, so switching the steriliser off during treatment is often advised. Follow the medication's instructions, and if in doubt ask a vet.

Use an earthed power strip kept dry and above the water level, and leave a drip loop in each cable below the socket so water cannot run down to it. A residual current device on the circuit is highly recommended.

Yes, unplug the electrical equipment before cleaning or changing anything in the tank, especially the heater and filter. It protects against electric shock if any device is damaged, and against the heater overheating when the water level drops.