Most fish disease in home aquariums is preventable. When it does turn up, most of it is one of about eight conditions, and most of those respond to treatments you can buy from any independent UK aquatic shop. This is the guide we wish we’d had before the first time we lost a fish for reasons we didn’t understand.

Read it symptom-first: find what your fish looks like in the identification table below, jump to that section for treatment, and then read the prevention section at the end — because knowing why disease turned up is worth more than knowing how to treat it.

Symptom-first identification

What you see	Most likely cause	Section
Pinhead white specks scattered on body and fins	Ich (white spot)	Ich
Fins ragged, edges eroding, sometimes with white edging	Fin rot	Fin Rot below
Fine gold-dust coating, especially in torchlight	Velvet	Velvet below
Cottony white patches on body	Body fungus (Saprolegnia)	Body Fungus below
Scales sticking out sideways, “pinecone” appearance	Dropsy	Dropsy below
White cottony mouth or jaw	Columnaris (cotton mouth)	Columnaris below
Fish swimming sideways, upside down, or unable to right	Swim bladder disease	Swim Bladder below
Small fish with fading colour, hiding, thin	Neon tetra disease	NTD below
Bloated belly, fish gasping at surface	Ammonia / nitrite poisoning (not disease)	Water Quality below
Fish flashing (rubbing on decor) but no visible marks	Early ich, or skin flukes	Ich and Flukes below

Fin Rot

What it is. Bacterial infection of fin tissue, usually secondary to physical damage or poor water quality. Fins fray, edges turn white or grey, and in severe cases the rot progresses to the body.

Treatment.

• First move: big water change (50%), test ammonia and nitrite. Fin rot in a properly cycled and clean tank is rare. Fix the water and mild cases often reverse on their own.
• If it’s advanced or spreading: treat with a UK antibacterial. NT Labs Anti-Fin Rot and Fungus, Waterlife Myxazin, or eSHa 2000 all work. Follow the label.
• Remove carbon from the filter before dosing; it strips medications.
• Continue for the full course even if fins look better after 3 days.

Prevention. Weekly water changes, no aggressive tank mates biting fins, no sharp decor.

Velvet (Oodinium)

What it is. Parasitic dinoflagellate. Coats the fish in a fine gold or rust-coloured dust that’s easiest to see in torchlight shone at an angle across the body. Fish scratch, breathe fast, and hide.

Treatment.

• Dim the lights. Velvet is photosynthetic — cutting light weakens it.
• Raise temperature to 28 °C over 24 hours.
• Dose with a copper-free treatment if the tank has shrimp or snails (eSHa EXIT works). If no invertebrates: Waterlife Protozin or a copper-based velvet treatment.
• Treat for 10 days minimum, even if visible signs stop after 4.

Prevention. Quarantine new fish for 3 weeks. Velvet almost always arrives on new stock.

Body Fungus (Saprolegnia)

What it is. Fungal infection, usually secondary to skin damage or poor water. Looks like cotton wool or a white/grey fuzz on the body or fins.

Treatment.

• Big water change, test water.
• Treat with an antifungal — NT Labs Anti-Fungus, Waterlife Myxazin, or salt bath (see below) for hardy species.
• Salt bath (short-term, hardy fish only): 3 g of aquarium salt per litre in a separate container, 5 minutes, then return to main tank. Do NOT do this with catfish, loaches, or scaleless species.
• Ensure tank has no ammonia and clean water throughout.

Prevention. Prevent injuries. Don’t over-crowd. Maintain clean water.

Dropsy

What it is. Not a disease itself but a symptom — fluid buildup in the body causing scales to protrude sideways. Usually caused by bacterial infection of the kidneys, sometimes by organ failure or heart disease.

Treatment.

• Realistically: dropsy is very hard to reverse. By the time you see the pinecone effect, internal organs are already failing. Prognosis is poor.
• If you want to try: isolate the fish in a hospital tank. Treat with an antibacterial medication (Waterlife Myxazin, NT Labs Anti Internal Bacteria, or Kanamycin if you can source it). Add Epsom salt at 1 teaspoon per 20 litres to help draw fluid out.
• Feed only high-protein food to preserve strength; the fish likely won’t eat much.

Prevention. Dropsy usually appears in fish that have had chronic poor water quality or stress. Keep water clean, don’t over-crowd, and quarantine new fish.

Columnaris (Cotton Mouth)

What it is. Bacterial infection, gram-negative. Named because it often shows first as a cotton-like fuzz on the mouth or jaw, but can also present as saddle-back lesions (a pale patch across the back).

Treatment.

• Move fast — columnaris can kill fish in 24–48 hours.
• Big water change and lower the temperature slightly (columnaris progresses faster in warm water).
• Treat with a broad-spectrum antibacterial — Waterlife Myxazin, NT Labs Anti Internal Bacteria, or eSHa 2000. If losses are ongoing after 48 hours, escalate to Kanamycin or Furan-2 (available from specialist UK sources, often needing to be imported).
• Isolate affected fish if possible.

Prevention. Columnaris arrives on new fish. Quarantine. Also thrives in warm, low-oxygen water — good filtration and reasonable temperatures reduce risk.

Swim Bladder Disease

What it is. Loose category for fish that can’t maintain buoyancy — floating at the top, sinking to the bottom, swimming sideways or upside down.

Common causes and fixes:

• Constipation — especially in fancy goldfish and bettas. Feed a blanched pea (skin removed, mashed) for 2–3 days. Withhold other food.
• Bacterial infection of the swim bladder — treat with a UK antibacterial like Waterlife Myxazin.
• Physical deformity — some fancy strains (fancy goldfish, balloon mollies) are born with compromised swim bladders. Not treatable.
• Ammonia damage — usually irreversible. Fix water and hope.

Prevention. Don’t over-feed. Feed sinking food to bottom-oriented species. Avoid heavily flake-based diets long-term.

Neon Tetra Disease

What it is. Sporozoan parasite (Pleistophora hyphessobryconis) that affects tetras, especially neons, cardinals, and rummy-nose. Fish lose colour along the sides, develop white patches under the skin, become thin, and swim erratically.

Treatment.

• There is no reliable treatment. By the time symptoms show, internal damage is extensive.
• Isolate and euthanise affected fish humanely to prevent spread to tank mates. Clove oil overdose is the standard UK method (available from pharmacies — 400 mg/l knocks the fish out, then a further dose overdoses it painlessly).
• Do not add new tetras to the tank for at least 6 months after the last case.

Prevention. Buy tetras from reputable sources. Quarantine. Cardinal tetras are less susceptible than neons.

Water Quality (Not Actually Disease)

What it looks like. Multiple fish gasping at the surface, red gills, bloated bellies, sudden deaths after a “nothing changed” event.

What’s actually happening. Ammonia or nitrite poisoning. Test water. If either is above zero, do a large water change immediately with dechlorinated water, and continue daily until readings are zero.

Most beginner “mystery deaths” are water quality. This is why proper cycling before adding fish matters more than any medication cabinet.

Skin and Gill Flukes

What they are. Microscopic worms attached to skin or gills. Fish scratch, breathe fast, sometimes have small red patches. Only diagnosable properly under a microscope but treat presumptively if symptoms are consistent and it’s not obviously ich.

Treatment.

• Praziquantel — most reliable, safe with most fish and invertebrates. Sold in the UK as Sera Tremazol or Fluke-Solve. Follow the label.
• Formalin-based treatments work but are harsher.

Prevention. Quarantine, don’t share equipment between tanks.

Prevention — the whole game

Nine tenths of the disease you’ll see in a home aquarium is preventable with these five habits:

1. Cycle the tank properly before adding fish. Ammonia and nitrite are more lethal than any pathogen.
2. Quarantine new fish for 3 weeks in a separate tank. Almost all disease enters your tank on new stock.
3. Weekly 25% water changes. Consistent water quality prevents stress; stress is what makes fish susceptible.
4. Don’t overstock. Waste production scales with fish load. Stressed, crowded fish get sick.
5. Buy from shops with visibly healthy stock. If you see one obviously sick fish in a shop tank, everything in that shop’s system is potentially contaminated.

Keep a hospital tank if you can — even a 30-litre with a sponge filter, ready to move a sick fish into. Treating individual fish in the main tank is often what breaks the tank’s cycle and causes more problems than it solves.

Products mentioned in this article

Frequently asked questions

What is the most common aquarium fish disease?

Ich (white spot), caused by the parasite Ichthyophthirius multifiliis, is the most common. It shows as pinhead-sized white specks and is treatable with heat and salt or proprietary medication.

How do I know if my fish is sick?

Watch for clamped fins, loss of colour, rapid gill movement, hiding, loss of appetite, visible spots or fluff, or flashing (rubbing on decor). Test water first — poor water quality causes most symptoms.

Should I treat the whole tank or just the sick fish?

For contagious diseases like ich, treat the whole tank. For a single injured or infected fish, isolate it in a hospital tank to protect sensitive species and invertebrates.

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The most common single disease we get emails about is ich (white spot) — we have a dedicated UK treatment guide for that. If you’re setting up a first tank, start with cycling, then our beginner stocking guide covers hardy species that get sick least often.

— Jaqline Smalling, FishForever