Common Aquarium Diseases:
Symptoms & Treatment

Prevention Is the Best Treatment

The vast majority of fish diseases are stress-related. A fish with healthy water parameters, appropriate tank mates, a good diet, and no stocking calculator has a strong immune system that resists most pathogens. Disease outbreaks most commonly follow water quality problems, temperature swings, overstocking, or the introduction of new fish without quarantine.

Before reaching for medication, always test your water. Ammonia, nitrite, nitrate, pH, and temperature should all be checked as a first step when any fish looks unwell. Many apparent "disease" symptoms — clamped fins, lethargy, loss of colour — are actually early stress responses to poor water quality that resolve with regular water changess and nothing else.

1. Ich (White Spot Disease)

Ich is the most common freshwater fish disease and the one most new aquarists will encounter. Caused by the parasite Ichthyophthirius multifiliis, it appears as tiny white spots on the fish’s body, fins, and gills — resembling grains of salt or sugar sprinkled over the fish.

Symptoms

• White spots (1–2mm) covering body, fins, and gills
• Fish rubbing against decorations or substrate (flashing)
• Rapid gill movement, gasping at the surface
• Loss of appetite and lethargy
• Clamped fins

Causes

Ich is almost always triggered by stress. Temperature fluctuations, introducing infected fish, or sudden changes in water parameters can suppress a fish’s immune system enough for the parasite — which exists in low levels in most aquariums — to take hold. Ich spreads rapidly once established.

Treatment

1. 1
   Raise the temperature

   Gradually increase tank temperature to 28–30°C (82–86°F) over 24 hours. Higher temperatures speed up the parasite’s life cycle, making it vulnerable to treatment faster. Do not do this with temperature-sensitive species like corydoras.

2. 2
   Add aquarium salt

   Dose 1 tablespoon of aquarium salt per 5 gallons. Salt disrupts the parasite’s osmotic balance without harming most fish. Avoid with scaleless fish (loaches) and sensitive species (corydoras, tetras).

3. 3
   Use an ich medication

   Products containing malachite green or copper (like Ich-X or API Super Ich Cure) are effective. Follow dosing instructions precisely and remove activated carbon from the filter before treating.

4. 4
   Continue for the full cycle

   Ich must be treated for at least 7–10 days even after spots disappear, because the free-swimming stage is what the medication kills. The spots you see are actually protected inside a cyst on the fish’s skin.

2. Fin Rot

Fin rot is a bacterial (or occasionally fungal) infection that eats away at a fish’s fins, causing them to appear ragged, discoloured, or progressively shorter. If untreated, it can spread to the fish’s body.

Symptoms

• Fins appear frayed, torn, or have a milky white or brown edge
• Dark or reddened edges on affected fins
• Progressive fin loss — fins become shorter over days
• Fish may become lethargic and lose appetite as infection advances

Causes

Poor water quality is almost always the root cause. Elevated ammonia or nitrate, overcrowding, or physical damage from fin-nipping tank mates create entry points for bacteria. Bettas are particularly prone to fin rot due to their long, delicate fins.

Treatment

Start with a thorough water change — 25–30% — and check all parameters. In mild cases, clean water alone allows fins to regrow. For moderate to severe cases, treat with an antibacterial medication such as API Melafix (mild) or API Furan-2 (stronger). Aquarium salt at 1 tablespoon per 5 gallons supports healing. Fins typically begin regrowing within 2–4 weeks of clean conditions, though the new growth may be slightly different in colour or shape.

3. Velvet (Gold Dust Disease)

Velvet is caused by the dinoflagellate parasite Oodinium and appears as an extremely fine golden or rust-coloured dust covering the fish — like someone sprinkled gold powder on it. It is more difficult to spot than ich and often progresses further before being noticed.

Symptoms

• Very fine gold, rust, or yellowish dust on body and fins (best seen under a torch at an angle)
• Rapid breathing, gasping at surface
• Fish rubbing against objects
• Lethargy, loss of appetite, clamped fins
• Skin may begin to peel in severe cases

Treatment

Velvet is more aggressive than ich and requires prompt treatment. Dim or completely darken the tank — the parasite is photosynthetic and light aids its reproduction. Raise the temperature to 28–29°C. Treat with a copper-based medication (API CopperSafe or Cupramine) for 14–21 days. Copper is toxic to invertebrates; remove shrimp and snails before treating. Do not use medications containing copper in a tank with live plants.

4. Dropsy

Dropsy is not a single disease but a symptom of internal organ failure, most commonly kidney failure caused by bacterial infection. It is characterised by severe bloating of the abdomen, causing the scales to protrude outward in what is often described as a “pinecone” appearance when viewed from above.

Symptoms

• Severely bloated abdomen
• Scales standing out from the body (pinecone appearance)
• Protruding eyes (exophthalmia)
• Pale gills, lethargy, loss of appetite
• Fish may float near the surface or at an angle

Prognosis and Treatment

Dropsy carries a poor prognosis once visible symptoms appear — by this point internal organs are already severely damaged. Immediate isolation of the affected fish is essential to prevent the spread of the underlying bacterial infection. Treatment with antibiotics such as Maracyn-2 (minocycline) or kanamycin may help in early cases. Aquarium salt (1 teaspoon per gallon) in the isolation tank reduces osmotic stress on the kidneys. Many experienced aquarists make the difficult decision to euthanise fish showing advanced dropsy, as recovery is uncommon.

5. Columnaris (Mouth Fungus)

Despite being called “mouth fungus,” columnaris is caused by the bacterium Flavobacterium columnare. It typically appears as white or grey patches around the mouth, head, or along the body, sometimes with ragged fins. It spreads rapidly, particularly in warm water.

Symptoms

• White, grey, or yellowish patches on mouth, head, or body
• Lesions may have a saddle-shaped appearance across the back
• Frayed fins with white edges
• Rapid breathing and lethargy

Treatment

Treat with an antibiotic — kanamycin or furan-based medications are most effective. Columnaris spreads quickly at high temperatures; reduce the tank temperature to 22–24°C during treatment if species allow. Clean, well-oxygenated water is essential — increase surface agitation during treatment. The acute form can kill fish within 24–48 hours, so act quickly.

6. Pop-Eye (Exophthalmia)

Pop-eye describes the abnormal protrusion of one or both eyes, caused by fluid build-up behind the eyeball. Like dropsy, it is a symptom of an underlying condition — usually bacterial infection, injury, or poor water quality — rather than a disease in itself.

Symptoms

• One or both eyes protruding noticeably beyond the normal position
• Eye may appear cloudy or have a white ring around the edge
• Fish may swim awkwardly due to altered vision

Treatment

If only one eye is affected, the cause is likely injury — the fish knocked into something. This form often resolves with clean water and salt treatment (1 tablespoon per 5 gallons). If both eyes are affected, it usually indicates a systemic bacterial infection; treat with Epsom salt (1 tablespoon per 5 gallons, separate from aquarium salt) to draw out fluid, combined with an antibiotic. A full water change and parameter correction is always the first step.

7. Swim Bladder Disorder

The swim bladder is an internal organ fish use to control their buoyancy. When it is damaged or impaired, fish lose the ability to maintain a normal position in the water column. They may float helplessly at the surface, sink to the bottom, or swim sideways or upside-down.

Symptoms

• Fish floating at the surface, unable to dive
• Fish sinking to the bottom, unable to rise
• Swimming at an angle, tilted, or upside-down
• Bloated or curved abdomen in some cases

Causes and Treatment

Common causes include overfeeding (leading to constipation and physical compression of the swim bladder), gulping air at the surface, bacterial infection, or physical injury. For goldfish and bettas, the most common cause is constipation. Try fasting the fish for 24–48 hours, then feeding a small piece of blanched, peeled frozen pea — the fibre often resolves the issue within a day or two. If caused by infection, treat with antibiotics. Raise the water level temporarily if a fish is struggling to reach the surface to breathe.

Quick Reference: Disease Identification

Summary

The pattern across almost every aquarium disease is the same: stress weakens the immune system, pathogens take hold, and disease follows. Pristine water quality, appropriate stocking, quarantine of new arrivals, and a good diet prevent the vast majority of problems. When disease does appear, act quickly — most freshwater diseases are treatable if caught early, and nearly impossible to reverse once advanced.

Keep a basic disease kit on hand: ich medication, an antibacterial treatment, Epsom salt, aquarium salt, and a liquid test kit. That kit, combined with a 10-gallon quarantine tank, is all you need to handle most emergencies confidently.