FAQ: Supplies for fish

A small round bowl with no filter or heater is unsuitable for most fish: it holds little water, oxygen exchange is poor and conditions fluctuate. Even goldfish and bettas need a larger tank with a filter.

Goldfish grow large and produce a lot of waste, so a tank of at least 100 litres is often recommended for a few of them. In a small container they grow poorly and get sick more often.

A betta is usually recommended a tank of at least 20 litres with a heater and gentle flow. In a small unheated bowl it lives a shorter life and gets sick more often.

Nano tanks are small aquariums, usually up to 30–40 litres, for shrimp, snails or a few small fish. They look great and take little space, but need careful care because water conditions change quickly in them.

Move the old filter or its media with the bacteria, plus some of the substrate and water, into the new tank. That way the new tank settles faster and the fish are less stressed.

A glass tank is heavier but more scratch-resistant and keeps its looks for a long time. An acrylic tank is lighter and less likely to break, but scratches easily, so it must be cleaned with soft tools made for acrylic.

It is glass with less iron, so it has no greenish tint and the view through it is clearer. The difference is most visible in thick, large tanks.

Internal filters are simple and suit small and medium tanks, external (canister) filters hold more media, run quieter and take no space inside the tank, and hang-on-back filters are easy to maintain. Sponge filters driven by an air pump are gentle and suit fry and shrimp.

Yes. Beneficial bacteria live in the filter and need a constant flow of water and oxygen. If the filter is switched off for a few hours they start to die and the water quickly gets worse.

Most tanks with a well-working filter that moves the water surface do not need a separate air pump. It is useful in heavily stocked tanks, during heatwaves, when treating fish or for running a sponge filter.

Mechanical media (sponges, floss) trap debris, biological media (ceramic rings, porous balls) give bacteria a place to live, and chemical media such as activated carbon remove odours and discolouration. They are usually layered.

Rinse the sponges in a bucket of tank water rather than tap water, because chlorine kills bacteria. Do not clean all the media at once, and do not clean the filter on the same day as a large water change.

Tropical fish, which need roughly 24–27 °C, require a heater because room temperature fluctuates. Coldwater fish such as goldfish usually do not need one.

A common rough rule is about 1 W per litre of water, a little more in a cooler room. The heater's packaging usually states the tank volume it is designed for.

Put the heater where the water moves, for example near the filter outlet or intake, so the heat spreads evenly. It must be submerged to the mark the manufacturer specifies.

Yes, a thermometer is very useful. A heater's thermostat can be inaccurate or fail, and a thermometer lets you notice in time that the water is too cold or overheating.

Keep the lid open, point a fan at the water surface, shorten the lighting period and close the curtains. Increase aeration, since warm water holds less oxygen, and use ice packs carefully so the temperature does not drop suddenly.

Most tropical fish do well at 24–27 °C, but some species, such as discus, need warmer water and others cooler. Most important is that the temperature is stable and suits all the fish kept together.

Usually 8–10 hours a day is enough. Longer lighting encourages algae growth, so a timer that switches the light on and off at the same times every day is handy.

LED lighting is the usual choice today: it is energy-efficient, runs cool and lasts a long time. If you will grow live plants, choose a light whose output and spectrum suit plants; a tank without plants needs only a simpler one.