Fish Diseases: Symptoms, Causes & Treatment

White Spot Disease (Ich)

Signs: Tiny white dots like grains of salt on the body and fins, flashing against objects, clamped fins and rapid breathing as the parasite reaches the gills.

Cause: The ciliate parasite Ichthyophthirius multifiliis, almost always brought in on a new fish or plant and triggered by a temperature drop or stress.

Treatment: Raise the temperature gradually to 28–30 °C (82–86 °F) to speed up the parasite's life cycle, add aquarium salt or a malachite-green/formalin medication, and keep treating for at least a week after the last spot disappears — the free-swimming stage is the only one medication can reach. Species guides for white spots (ich) →

Fin Rot in Fish

Signs: Fin edges turn milky, ragged or black and recede toward the body; in advanced cases the base of the fin reddens and the rot reaches the body.

Cause: Opportunistic bacteria (Aeromonas, Pseudomonas, Flavobacterium) that take hold when ammonia, nitrite or nitrate is high, or after fin-nipping from tank mates.

Treatment: Test the water and do a 30–50 % water change first — mild fin rot heals on clean water alone. Persistent cases need a broad-spectrum antibacterial in a hospital tank, and the nipping tank mate has to go. Species guides for fin rot →

Fungal Fish Diseases (Saprolegnia)

Signs: Cotton-wool tufts, white or grey, on the body, mouth or fins, usually growing over an existing wound or patch of missing scales.

Cause: Water moulds that are always present in the tank and only colonise damaged tissue — so a fungal outbreak means something injured or weakened the fish first.

Treatment: Move the fish to a hospital tank, treat with an antifungal (methylene blue or a malachite-green product) and fix the original injury or water problem. Cotton-mouth that does not respond is usually columnaris, a bacterium, and needs an antibacterial instead. Species guides for fuzzy white growth →

Dropsy in Fish

Signs: A swollen abdomen with scales standing out like a pinecone, often with bulging eyes, pale gills and lethargy.

Cause: Not one disease but a symptom of internal organ failure, most often a bacterial infection of the kidneys after long-term poor water quality or an internal parasite load.

Treatment: Isolate immediately, add Epsom salt (1 tsp per 5 gallons) to draw off fluid, keep the water pristine and try a medicated food or antibacterial. Once the pinecone effect is obvious the prognosis is poor, so this is a disease to prevent rather than cure. Species guides for bloated or swollen →

Velvet Disease (Oodinium)

Signs: A gold or rust-coloured dust over the body and fins that is easiest to see with a torch in a dark room; fish flash, clamp their fins and breathe hard.

Cause: The dinoflagellate parasite Oodinium (Piscinoodinium in freshwater, Amyloodinium in marine tanks). It spreads faster than ich and kills small fish quickly.

Treatment: Dim the tank — the parasite photosynthesises — raise the temperature, and treat with copper or a malachite-green/acriflavine medication for the full course. Remove carbon from the filter while medicating. Species guides for spots on fins →

Popeye in Fish

Signs: One or both eyes swell outward from the socket; the eye may cloud over and the fish stops eating.

Cause: One bulging eye is usually an injury; both eyes point to a systemic bacterial infection or poor water quality, and it is a common companion of dropsy.

Treatment: Clean water and Epsom salt resolve most injury cases in a week or two. Bilateral popeye needs an antibacterial in a hospital tank and a hard look at nitrate levels. Species guides for bulging eyes (popeye) →

Swim Bladder Disease

Signs: The fish floats at the surface, sinks to the bottom or swims sideways and cannot hold a normal position, but otherwise looks alert.

Cause: Usually constipation or gulped air from overfeeding dry food, especially in round-bodied goldfish and bettas; less often an infection, injury or a birth defect.

Treatment: Fast the fish for two or three days, then feed a shelled, blanched pea or daphnia to clear the gut, and soak dry food before feeding from then on. If a fast does not help within a week, treat as a bacterial infection. Species guides for swimming sideways →

Columnaris Disease in Fish

Signs: A white or grey saddle-shaped patch across the back, fuzzy lips, excess slime coat, frayed fins and rapid gill movement — it looks like fungus but moves much faster.

Cause: The bacterium Flavobacterium columnare, which thrives in warm, hard water with a high organic load and attacks stressed or overcrowded fish.

Treatment: Lower the temperature a few degrees if the species allows, do large water changes and treat with an antibacterial that targets gram-negative bacteria (nitrofurazone, kanamycin) in a hospital tank. Fast-moving cases can kill within 48 hours, so start the same day. Species guides for excess mucus coating →

What are the most common fish diseases in aquariums?

White spot disease (ich), fin rot, fungal infections, velvet, dropsy, popeye, columnaris and swim bladder disorder account for the large majority of cases hobbyists see. Ich and fin rot alone are behind most of the questions submitted to the FishCare AI symptom checker.

Is my fish sick or just stressed?

Stress shows as behaviour — hiding, clamped fins, faded colour, refusing food — with nothing visibly growing on the fish, and it improves within a day or two of fixing the cause. Fish diseases add physical signs that get worse over time: spots, fuzz, rot, swelling or red streaks. Test the water first; if it is fine and the sign is spreading, treat it as disease.

Can fish diseases spread to other fish in the tank?

Most infectious fish diseases — ich, velvet, columnaris, fin rot bacteria, fungus — spread through the water and will reach every fish in the tank. Move the sick fish to a hospital tank as soon as you spot a symptom, and treat the main tank too if the disease is parasitic.

Can aquarium fish diseases infect humans?

Very few. Fish tuberculosis (Mycobacterium marinum) can cause a skin infection through a cut on your hand, and Aeromonas can infect open wounds. Wear gloves if you have a cut, wash your hands after tank work, and never start a siphon with your mouth.

How long do fish diseases take to cure?

Mild fin rot and stress-related symptoms clear within a week of clean water. Ich needs 10–14 days of treatment to cover the parasite's full life cycle. Bacterial infections typically need a 5–10 day course of medication. Dropsy and advanced columnaris are often fatal even when treated, which is why prevention matters more than any medicine.

Which fish diseases does aquarium salt treat?

Salt at 1–3 teaspoons per gallon helps with ich, mild fungus, fin rot and external parasites, and Epsom salt helps with dropsy, popeye and constipation. Salt does not treat internal bacterial infections, and scaleless fish such as corydoras, loaches and plecos tolerate it poorly — use half strength or a medication instead.