Common aquarium fish diseases: ich, fin rot and fungus signs and treatment

TUREFF• •0

Most fish diseases are linked to stress, and the biggest source of stress is poor water quality. That makes the first step for almost any sick fish the same: test the water. This guide covers three of the most common problems, how to recognise them and the general approach to treating them. It is no substitute for advice on your particular fish.

Start with the water

Before reaching for medicine, test for ammonia, nitrite, nitrate, pH and temperature. Ammonia or nitrite above zero, a temperature swing or a recently overstocked tank can cause or worsen almost every disease below. A partial water change with temperature-matched, dechlorinated water is often the right first move. Medication rarely works well in poor water.

Ich (white spot)

Ich is caused by a parasite and is one of the most common aquarium diseases, often arriving with new fish.

Signs: small white spots, like grains of salt, on the body and fins; fish rubbing against objects ("flashing"); clamped fins; fast breathing.
Why it is tricky: the visible spots are the parasite buried under the skin, where most treatments cannot reach it. Treatments work against the free-swimming stage after it drops off the fish, so treatment has to continue for some time after the spots disappear.
Treatment in general: commercial white spot treatments are widely available. Some keepers also raise the temperature gradually, since warmth speeds up the parasite's life cycle. Not all species tolerate higher temperatures, and warm water holds less oxygen, so add aeration and check what your fish can handle first.

Fin rot

Fin rot is a bacterial infection, usually linked to poor water quality, stress or injury from fin-nipping tankmates.

Signs: fins look ragged, frayed or shorter; edges may be white, red or dark; in bad cases the rot reaches the body.
Treatment in general: fixing water quality is often the most important step, and mild cases can improve with clean water alone. More advanced cases may need a suitable medication. Check for and separate any fin-nipping tankmates.

Fins that are split cleanly from a fight or snag and are otherwise healthy are not fin rot, though they can become infected.

Fungus

Signs: white or grey cotton-wool-like tufts on the body, fins or mouth, often where there is already a wound.
Treatment in general: commercial antifungal treatments are available. Treat the cause as well, whether injury, stress or poor water. Fluffy growth around the mouth can also be a different, bacterial condition, which is one reason a firm diagnosis matters.

Quarantine tanks

A small separate tank with a filter, heater and a hiding place is one of the best investments in fish health.

Quarantine new fish for a few weeks before adding them to your main tank. Many keepers allow at least two weeks; some sources recommend longer.
Use it as a hospital tank to treat a sick fish without dosing the whole aquarium, which can harm plants, invertebrates and the filter bacteria.
Keep a sponge running in the main filter so the quarantine tank can be cycled quickly when needed.

Using medication safely

Follow the manufacturer's instructions exactly, including dose, duration and water changes between doses.
Calculate the real water volume, which is less than the tank's nominal size once substrate and decor are in.
Remove chemical filtration such as activated carbon during treatment if the instructions say so, as it can remove the medicine.
Do not mix medications unless the instructions say it is safe.
Check warnings for scaleless fish, shrimp, snails and plants, which can be sensitive.
Increase aeration during treatment.

When to get specialist advice

Seek help from a vet with fish experience or an experienced aquatic specialist when:

fish keep dying despite good water quality
the problem does not respond to treatment, or you are unsure what it is
you see ulcers, swollen bodies, raised scales, bulging eyes or unusual growths
a valuable or long-lived fish is affected

In some countries there are vets who specialise in fish, and they can carry out tests that home keepers cannot. Has your tank been through a disease outbreak? Share what you learned or ask about symptoms you are seeing in the comments below.