FAQ: Health for fish

Cloudy or bulging eyes are often linked to poor water quality, injury or infection. In some breeds, such as certain fancy goldfish, bulging eyes are natural. If the change appeared suddenly, test the water and talk to a specialist.

Balance problems are often related to the swim bladder, especially in goldfish, and can come from feeding, water quality or illness. Test the water, review the feeding, and see a vet if the fish does not improve within a day or two.

Fish naturally go pale at night, when frightened or after moving to a new tank. If the colour stays faded longer, stress, poor water, unsuitable food or illness may be the cause. Test the water and watch for other signs.

A fish may stop eating because of stress, too low a temperature, poor water, unsuitable food or illness. First check the water parameters and temperature, and try a different food. If it refuses food for several days and shows other signs, see a specialist.

Rapid breathing shows the fish is short of oxygen or its gills are damaged, for example by ammonia, nitrite, high temperature or parasites. Test the water and increase aeration. If that does not help, see a vet.

Long, white, slimy droppings can point to a digestive problem or internal parasites, but they also appear when a fish has not eaten for a while. Watch whether the fish eats and keeps its weight. If the sign persists, see a vet.

Some species jump naturally, but frequent jumping can point to poor water, stress or being chased by other fish. Check the water parameters and compatibility, and keep the tank covered with a lid so fish cannot leap out.

Sudden frantic swimming can point to toxic substances in the water, such as chlorine after a water change without conditioner, a temperature spike or a sudden change in parameters. Test the water straight away and, if needed, change part of it with properly prepared water.

Some species are naturally shy or active at night. If a normally bold fish starts hiding constantly, aggression from others, too small a group, harsh light, too few hiding places or illness may be the cause. Watch their behaviour and test the water.

Red spots, streaks or reddened gills and fin bases can point to irritation from ammonia or nitrite, or to infection. Test the water straight away and change part of it. If the redness does not fade or spreads, see a vet.

When a fish develops a greyish or bluish film of slime, its skin is being irritated: poor water, a sudden change in parameters or parasites may be to blame. Test the water and consult a specialist, because this condition can worsen quickly.

Ageing fish slow down gradually, may fade a little and eat less, but this happens slowly over a long time. Sudden changes, spots, film, wounds or breathing problems are not signs of age, so test the water and consult a specialist then.

Water is both air and home to a fish, so poor quality causes constant stress and weakens immunity. Many fish diseases start with unsuitable water, so regular testing and water changes are the most important prevention.

Ammonia forms from fish waste and rotting food and damages gills and skin even in small amounts. It usually rises in a new tank, when there are too many fish, when they are overfed or when the filter stops working properly.

Nitrite is an intermediate product of ammonia breakdown and stops fish blood carrying oxygen, so fish can suffocate even in well-oxygenated water. It is most common in new tanks or after cleaning the filter too thoroughly.

Nitrate is far less toxic than ammonia and nitrite, but high levels that build up over time stress fish, weaken immunity and feed algae. Regular water changes, live plants and moderate feeding help keep it low.

Change part of the water with conditioned water, feed less or not at all for a few days and check that the filter works. Retest the next day and repeat the change if needed until the readings fall. Look for the cause: a dead fish, overfeeding or too many fish.

It is when too many fish go into a new, uncycled tank before the filter has enough bacteria. Ammonia and nitrite then build up and fish fall ill or die. Cycling the tank in advance and adding fish gradually prevents it.

Many fish adapt to a fairly wide pH range, but sudden pH changes stress them greatly. Stability therefore usually matters more than an exact number. pH also determines how much ammonia is present in its most toxic form.

Usually not. pH-adjusting products cause sudden swings that are more dangerous for fish than a stable, slightly less than ideal pH. It is generally easier to choose fish that suit your water, and if a change is necessary, make it gradually after talking to an experienced aquarist.