Freshwater Fish Diseases: Complete Guide to Diagnosis & Treatment

Also known as: White Spot Disease

Freshwater Ich is the single most common disease in freshwater aquariums worldwide. Nearly every fishkeeper will encounter it at some point. It is caused by the protozoan parasite Ichthyophthirius multifiliis and is highly contagious.

Symptoms

• Distinct white spots resembling grains of salt on the body, fins, and gills
• Fish rubbing against rocks, gravel, or decorations (flashing)
• Clamped fins and reduced activity
• Rapid gill movement and breathing difficulty
• Loss of appetite

Treatment

• Raise water temperature gradually to 30°C (86°F) to speed up the parasite's life cycle
• Add aquarium salt — 1 tablespoon per 5 gallons (not for scaleless fish like corydoras or loaches)
• Ich medication — products containing malachite green or formalin (API Super Ick Cure, Sera Costapur)
• Continue treatment for at least 7-10 days after the last visible spot disappears
• Perform 25% water change before each dose of medication

Caused by: Aeromonas bacteria (usually secondary to organ failure)

Dropsy is not a disease itself but a symptom of internal organ failure, usually kidney failure. It causes the fish to swell dramatically as fluid accumulates inside the body. Unfortunately, dropsy has a very low survival rate by the time symptoms appear.

Symptoms

• Pinecone-like appearance — scales standing outward from the body (pathognomonic sign)
• Severely bloated or swollen abdomen
• Bulging eyes (popeye)
• Pale or discolored gills
• Lethargy and loss of appetite
• Stringy, white feces

Treatment

• Isolate in a quarantine tank immediately
• Epsom salt bath — 1 tablespoon per 5 gallons to help draw out excess fluid
• Antibiotic food — Kanaplex or Maracyn-2 mixed into food
• Maintain pristine water quality — daily 25% water changes in the quarantine tank
• Feed high-quality food with garlic guard to boost appetite

Prognosis

If the scales are already pineconing, the prognosis is very poor (less than 20% survival). The key is catching it before the pinecone stage — watch for early signs like bloating and lethargy.

Also known as: Swim Bladder Disorder, Flip-over Disease

Swim bladder disease affects the fish's buoyancy organ, causing it to float abnormally, sink to the bottom, or swim sideways/upside down. It is particularly common in fancy goldfish, bettas, and balloon-bodied fish varieties.

Symptoms

• Fish floating at the surface unable to swim down
• Fish sinking to the bottom unable to swim up
• Swimming sideways, upside down, or in spirals
• Bloated belly (if caused by overfeeding or constipation)
• Curved spine in severe cases

Causes

• Overfeeding or constipation — the most common cause
• Swallowing air while eating floating food
• Bacterial infection of the swim bladder
• Physical injury from fighting or poor handling
• Genetic predisposition — fancy goldfish with compact bodies are prone

Treatment

• Fast the fish for 2-3 days — this resolves most cases caused by overfeeding
• After fasting, feed blanched, de-shelled peas — a natural laxative for fish
• Lower the water level to reduce swimming effort
• If bacterial, treat with Kanaplex or Maracyn-2
• Switch to sinking pellets to prevent air gulping

Also known as: Cotton Mouth Disease, Mouth Fungus, Saddleback Disease

Columnaris is a bacterial infection often mistaken for a fungal infection due to its cotton-like appearance. It is highly contagious and can kill fish within 24-48 hours in its acute form. It thrives in warm water (above 28°C), making it particularly relevant for Indian aquariums.

Symptoms

• White or greyish-white patches on the mouth, body, or fins
• Cotton-like growths, especially around the mouth
• Saddle-shaped pale patch on the back (saddleback lesion)
• Frayed or eroded fins
• Rapid breathing and lethargy
• Loss of appetite

Treatment

• Lower water temperature to 24°C (75°F) — Columnaris bacteria multiply faster in warm water
• Antibiotics: Kanaplex + Furan-2 combination is the most effective treatment
• Add aquarium salt (1 tablespoon per 5 gallons) to help with osmotic stress
• Methylene blue baths for external lesions
• Treat for at least 10 days even if symptoms improve

Caused by: Saprolegnia, Achlya, and other water molds

Fungal infections in freshwater fish typically appear as white, cotton-like growths on the skin, fins, or gills. Fungi are opportunistic — they usually infect fish that are already weakened by injury, stress, or another disease.

Symptoms

• White, fluffy, cotton-like growths on the body or fins
• Growths on the eyes (fungal eye infection)
• White patches on eggs (egg fungus during breeding)
• Affected areas may become red or inflamed underneath the fungus

Treatment

• Methylene blue — effective antifungal, safe for most fish (will stain decorations)
• API Pimafix — milder antifungal treatment
• Salt baths — 1 tablespoon per gallon for 15-30 minutes (not for scaleless fish)
• Potassium permanganate dip — for severe cases (use with caution, proper dosing critical)
• Identify and treat the underlying cause (wound, stress, poor water quality)

Also known as: Gold Dust Disease, Rust Disease

Freshwater velvet is caused by the parasitic dinoflagellate Oodinium pilularis. It appears as a fine, gold or rust-colored dust on the fish's skin and is easily missed in early stages. Like its marine counterpart, it primarily attacks the gills and can be lethal if untreated.

Symptoms

• Fine gold, rust, or yellowish dust on the skin (best seen with a flashlight at an angle)
• Rapid breathing and gasping
• Clamped fins
• Loss of appetite and lethargy
• Scratching against objects
• Skin peeling in advanced cases

Treatment

• Copper medication (Cupramine or Copper Safe) — most effective treatment
• Dim the lights or blackout the tank — Oodinium relies on photosynthesis
• Raise temperature to 28-30°C to speed up the parasite's life cycle
• Add aquarium salt (1 tsp per gallon) as supplemental treatment
• Treat for a full 14 days minimum

Caused by: Lernaea copepod parasite

Anchor worms are crustacean parasites that burrow their anchor-shaped heads into the fish's flesh. They are visible to the naked eye as thin, thread-like worms protruding from the fish's body. They are particularly common in pond fish and goldfish but can infect any freshwater species.

Symptoms

• Thin, whitish-green worms (5-20mm long) protruding from the body
• Red, inflamed spots at the attachment site
• Fish rubbing against objects trying to dislodge the parasite
• Ulcers forming at attachment points
• Secondary bacterial infections at wound sites

Treatment

• Manual removal — gently pull the worm out with tweezers, then dab the wound with iodine or Melafix
• Dimilin or Cyromazine — kills juvenile anchor worms in the water column
• Potassium permanganate bath — 10mg/L for 30 minutes to kill parasites
• Treat the entire tank since juveniles are free-swimming
• Repeat treatment every 7 days for 3-4 weeks to catch all life stages

Caused by: Bacterial infection, injury, or poor water quality

Popeye causes one or both eyes to bulge outward from the socket. It can affect any freshwater fish species. If only one eye is affected, it's usually caused by physical injury. If both eyes are affected, it typically indicates a systemic bacterial infection or severe water quality problems.

Symptoms

• One or both eyes protruding abnormally from the head
• Cloudiness or haze on the eye surface
• Swelling around the eye socket
• In severe cases, the eye may rupture or fall out

Treatment

• Improve water quality immediately — perform 50% water change and test all parameters
• Epsom salt — 1 tablespoon per 5 gallons to reduce swelling
• Antibiotics — Kanaplex or Maracyn-2 for bacterial popeye
• For injury-caused popeye, clean water and time are usually sufficient
• Recovery takes 1-3 weeks with proper treatment