FAQ: Health for fish

Many freshwater plants are sensitive to salt, so prolonged use can make leaves melt or kill plants. When salt is needed, it is more often used in a separate hospital tank without plants.

Salt does not evaporate, so it only leaves the tank through water changes. Do not add salt when topping up evaporated water, and after treatment do several partial changes with unsalted water so the salt level drops gradually.

No. Salt is sometimes used as a supporting measure, but it does not cure most diseases and can harm sensitive species. If fish are sick, see a vet, who will advise whether salt is suitable and whether other measures are needed.

They are short procedures in which a fish is briefly placed in a stronger salt solution. Done wrongly, they can be very dangerous for the fish, so they are only worth doing when a vet has advised it and explained how.

First dissolve the salt in a separate container of tank water, then pour it in gradually, away from the fish. Do not pour dry salt straight into the tank, as undissolved crystals on the bottom can irritate bottom-dwelling fish. Use the amount on the label or given by a specialist.

Pond keepers sometimes use salt as a supporting measure, but it harms many pond plants and does not evaporate, so it builds up. Whether and how to use it is worth discussing with a vet rather than adding it routinely.

Most popular livebearers, such as guppies or platies, do not need salt: clean, moderately hard water is usually enough for them. Salt is only used routinely for species that live in brackish water in nature.

Keep it tightly closed in a dry place so it does not absorb moisture and clump, and clearly labelled so nobody confuses it with other products or kitchen salt. Store it out of children's reach.

The essentials: stable, clean water, regular partial changes, moderate and varied feeding, no overstocking, compatible species and quarantine for new fish. Watching the fish daily lets you spot problems early while they are still small.

The commonest causes are poor or unstable water, the wrong temperature, aggressive tank mates, overcrowding, too few hiding places, too small a group of schooling fish, sudden noises and frequent disturbance. Long-term stress weakens immunity and invites disease.

Incompatible species may nip each other's fins, chase each other or stop others feeding, and differing temperature and water needs mean conditions will not suit at least one of them. Before buying new fish, check that they suit your tank's residents.

Schooling fish, such as many tetras and danios, live in large shoals in nature and feel unsafe alone or in a small group. They become stressed, hide, go pale and fall ill more easily, so keep them in a group of the size recommended for the species.

Caves, rocks, wood and dense plants let fish hide, rest and avoid more aggressive tank mates. Feeling safe, fish are less stressed, show brighter colours and often behave more boldly.

Yes. A tank by a door, speakers, a TV or a walkway is often disturbed, and by a window or radiator the temperature fluctuates. A quieter spot away from heat sources and direct sun causes the fish less stress.

Yes, knocks on the glass travel through the water and frighten the fish, and repeated tapping causes them constant stress. Explain to children and visitors that fish are best watched quietly without touching the tank.

Fish need a steady day and night rhythm and a dark period to rest. Lights left on all the time or switched randomly cause stress. A timer helps keep the same schedule every day.

Chased fish are under constant stress, and bite wounds and torn fins become entry points for infection. Aggression is therefore a health problem as well as a behaviour one, and worth dealing with early.

Try rearranging the decorations to break up territories and add hiding places and sight barriers of plants or rocks. If that fails, the aggressor or the victim will need separating. Check whether the species are suitable to keep together at all.

An injured fish most needs very clean, stable water and peace, so it can be moved to a separate tank if needed. Watch whether the wound grows film or gets bigger. If the wound is deep or worsening, see a vet.

Yes, especially long-finned and veil-tailed fish. Before adding a decoration, run your hand or a thin piece of fabric over it: if the fabric snags, the edge could tear fins. Smooth sharp edges or choose smooth decorations.