FAQ: Health for fish

A healthy fish is active, swims evenly, has spread, untorn fins, bright colour, clear eyes and a body free of spots, film and wounds. Ask to see the fish feed and look over the whole tank, not just the fish you picked.

Better not. If the same tank or system holds dead or obviously sick fish, even healthy-looking ones may be infected. It is safer to wait a few days or choose fish from another tank.

Take the fish home as quickly as possible, keeping the bag dark in a cool box or carton so the water does not cool or heat too much. Do not leave the bag in the sun or a cold car, and do not shake it, as that stresses the fish.

First float the bag with the fish in the tank for about a quarter of an hour so the temperatures match. Then add small amounts of tank water to the bag several times so the fish gradually adjusts to the new parameters, and only then move it with a net.

The fish and its bag water go into a clean container, and tank water is dripped in slowly through thin tubing. The water changes very gradually, so this method is often used for sensitive fish and shrimp.

Better not. Shop water can contain pathogens, snails or built-up ammonia. Move the fish into the tank with a net or pour it out carefully through a net, and discard the bag water.

The move and the new surroundings stress fish, so in the first days they often hide and eat little. This usually passes within a few days. If a fish refuses food for long, breathes heavily or shows other signs, test the water and ask a specialist.

It is often advised to keep the tank dim for the first few hours. In calmer surroundings new fish settle faster and resident fish are less likely to chase them. Start feeding the next day, in small amounts.

The commonest causes are an uncycled tank where ammonia and nitrite build up, transport stress, a sudden change of water or a disease brought home. Water tests help rule out a water problem, and for diseases it is worth consulting a vet.

Remove the dead fish straight away, test the water and watch the other fish closely. It is worth telling the seller, and if other fish show signs too, contact a vet.

Behaviour usually changes first: the fish eats less, hides, stays apart from others, clamps its fins, or its colour or breathing changes. When you notice such changes, test the water first, and if the signs persist, consult a specialist.

For some species, such as bottom dwellers, resting on the bottom is natural. If a normally active fish lies on the bottom for long, it can point to poor water quality, the wrong temperature, stress or illness. Test the water and see a vet if it does not improve.

It usually points to low oxygen, too high a temperature, or excess ammonia or nitrite damaging the gills. Test the water straight away, increase aeration and change part of the water. If that does not help, a gill disease needing a specialist is possible.

An occasional rub is usually nothing, but frequent rubbing means something is irritating the skin or gills: poor water or parasites. Check the water parameters, and if the rubbing continues or spots appear, see a vet.

Tiny white spots like grains of salt are often linked to a parasitic disease, but other things can look similar, such as breeding tubercles on some fish. To choose the right treatment, it is worth confirming the cause with a vet.

White or greyish cotton-like growth usually points to an infection, which often develops on an injury or on a weakened fish. Separate the fish, test the water and ask a vet about suitable treatment.

Torn fins are often caused by fin-nipping from other fish or sharp decorations, while ragged, eaten-away edges can point to an infection favoured by poor water quality. Watch how the fish behave, test the water, and see a specialist if the fins keep deteriorating.

Clamped fins are a common general sign of stress or illness. Poor water, the wrong temperature, aggressive tank mates or disease can cause it. Test the water parameters and temperature first, and see a specialist if it does not improve.

A swollen belly can be due to overfeeding, eggs in a female or an internal illness. If the fish otherwise behaves normally, feed sparingly for a few days and watch. If the belly keeps swelling, the fish stops eating or the scales lift, see a vet.

When the scales stand out and the fish looks like a pine cone, it usually signals a serious internal problem causing fluid to build up in the body. It is a serious sign: separate the fish, test the water and see a vet as soon as possible.