Most aquarium diseases do not begin with an "infection brought in from outside" but with conditions: a swing in parameters, overcrowding, ammonia after overfeeding, a chill during a water change. The pathogens were usually in the tank already — a healthy fish was simply coping with them. That is why treatment nearly always starts with the water, not with a medication.

The first signs

It is worth worrying when a fish:

  • clamps its fins, hangs near the bottom or at the surface;
  • refuses food or spits it out;
  • breathes rapidly, gulping at the surface while aeration is running;
  • scratches against the substrate and decor;
  • darkens or, on the contrary, pales and loses its sheen;
  • develops spots, a film, slime or ulcers;
  • raises its scales, bulges an eye, swells up.

Behaviour changes before anything visible appears. A fish that was first to the food yesterday and stands in a corner today is already reason enough to test the water.

What to do first

  1. Test the water: ammonia, nitrite, nitrate, temperature, pH. Ammonia and nitrite must be zero. Poisoning by them looks like "a disease" and is cured by water changes, not by medication.
  2. Change 25–30 % of the water, matching temperature and parameters, and remove leftover food and debris from the substrate.
  3. Increase aeration. A sick fish needs more oxygen, and warmer water holds less of it.
  4. Take carbon and other adsorbents out of the filter if you intend to medicate: they will strip the treatment out of the water.
  5. Look at the others. One sick fish among ten healthy ones is more often injury or old age than an epidemic.

Common diseases

Ich (white spot). White grains the size of semolina over the body and fins, and the fish scratches. The parasite is vulnerable only in its free-swimming stage, so the course is finished even after the spots have gone. Raising the temperature speeds up the parasite's cycle. But it can only go as high as the species tolerates: the upper limit is given in the species profile, and for many soft-water fish it is below the "standard" 30 °C.

Velvet (gold dust). A fine dust over the body, as if the fish had been sprinkled with flour; the gills suffer before the skin does. The tank is shaded, because the parasite needs light, and treated with a purpose-made medication.

Fin rot. The edges of the fins turn cloudy, fray and shorten. Almost always the result of poor water, an injury or nipping by tank mates. Clean water and removing the cause do more than any medication.

Fungus (cotton wool disease). White fluffy tufts on the body. Always secondary: it settles on a wound, a bite or already dead tissue. It is the underlying cause that has to be treated.

Hexamita (hole-in-the-head). Ulcers on the head and along the lateral line, white stringy faeces, refusal of food. Most often in cichlids, especially with a monotonous diet and chronic stress.

Dropsy (raised scales). A swollen body with scales sticking out like a pine cone. This is not a disease but a sign of organ failure; the prognosis is poor and treatment rarely helps.

Rules of treatment

  • Do not mix medications and do not "reinforce" the dose. An overdose kills faster than the disease.
  • Follow the product instructions — the dose, the duration, the water changes between applications.
  • Be careful with scaleless fish. Catfish, loaches, rays and fry tolerate medications worse: their dose is usually reduced, and copper-based products need particular caution.
  • Treat where the disease is. A hospital tank suits a single fish, but if the parasite has already been through the whole aquarium, the whole tank has to be treated.
  • Salt is not a universal remedy. Many freshwater species tolerate it badly; the acceptable level is given as a separate parameter in each species profile.
  • Do not stop the course early. Symptoms disappearing is not recovery but the middle of the parasite's cycle.

Prevention

Quarantining new fish for two to four weeks is the one measure that reliably protects an aquarium. Water from the shop bag never goes into the tank. Simple things help too: do not overfeed, do not overstock, change water regularly, keep the temperature stable, and do not move fish between tanks with a shared net.

A separate source of infection is live food from unchecked waters. Frozen or cultured food is safer.

When not to treat

Cloudy eyes in an old fish, a torn fin after a fight, faded colour after a move — none of these calls for medication. Chemistry in an aquarium is a stress in itself: it hits the biofilter and the fish's mucous coat. If the water parameters are fine and the fish eats and behaves normally, it is wiser to wait and watch.