FAQ: Supplies for fish

No, fish need darkness to rest. A night or blue light can be used briefly for viewing, but a light left on all night disrupts the fishes’ rhythm and encourages algae.

LED lights last many years, although their brightness slowly fades. Older fluorescent tubes are often recommended to be replaced about once a year, because their spectrum shifts and plants stop getting enough light.

Gravel is easy to clean and suits most fish, sand is needed for bottom-sifting fish such as corydoras, and nutrient-rich substrate is meant for densely planted tanks. Choose a substrate without sharp edges and rinse it well before use.

A layer of 4–7 cm is usually recommended, a little deeper for plants with strong roots. The packaging usually states what floor area or tank volume the substrate covers.

Not essential, but live plants absorb nitrate, compete with algae and give fish places to hide. Undemanding plants such as anubias, Java fern or cryptocorynes suit beginners.

Yes – they need no light or fertiliser, do not wilt and do not get eaten. Choose soft plastic or silk plants without sharp edges, especially if you keep long-finned fish, and wipe the film off them now and then.

Decor, rocks and wood made for aquariums are the safest. Items from nature or around the house can release harmful substances, change water hardness or have sharp edges, so leave them out unless you are sure they are safe.

Hiding places let fish rest and get away from more aggressive tankmates, so they are less stressed. Catfish, cichlids and nocturnal fish need them most; caves, wood, rocks and dense plants all work.

Yes, but it must be treated with a water conditioner that neutralises chlorine and chloramine. The water you add should be at a similar temperature to the tank.

Chlorine and chloramine in tap water damage fish gills and kill filter bacteria. A conditioner neutralises them, and some also bind heavy metals; dose according to the label.

To start with, ammonia, nitrite, nitrate and pH tests are the most useful, and hardness (GH, KH) tests help if you keep more demanding species. Strips are handy for quick checks, while liquid drop tests are usually more accurate.

In a new tank it is worth testing every few days until the cycle is complete. In an established tank once every week or two is enough, plus whenever the fish behave unusually.

pH shows whether water is acidic or alkaline. Most common aquarium fish do well at roughly 6.5–7.5, and most important is that pH does not swing suddenly; it is worth changing only when particular species need it.

Hardness shows how many dissolved minerals, mainly calcium and magnesium, the water contains. Some fish, such as Malawi cichlids, prefer hard water and others soft, so choose species that suit your water.

RO (reverse osmosis) water has had almost all minerals removed. It is used by keepers of soft-water fish, shrimp or marine tanks, but before use it must be remineralised to the right parameters.

In an established tank ammonia and nitrite should be zero or close to zero. If tests show them, do a partial water change, feed less and check the filter.

Nitrate is less harmful but builds up over time. It is often recommended to keep it below roughly 20–40 mg/l through regular partial water changes and live plants.

The often-quoted rule of ‘1 cm of adult fish per litre of water’ is only a very rough guide. Body mass, activity, filter capacity and whether the species needs a group also matter, so it is better to understock than overstock.

Before buying, find out how big the species grow, what water they need and how they behave. Large predatory fish can eat small ones, and some species nip other fish’s fins.

Shoaling fish such as neons, rasboras or corydoras feel insecure alone or in pairs, hide more and look paler. A group of at least 6–10 is often recommended for them.