Common Fish Diseases UK: Ich, Fin Rot, and Velvet Treatment Guide

What Is the White Spot Disease (Ichthyophthirius Multifiliis)?

White spot, commonly called ich, is the most prevalent disease in UK aquariums. It is caused by the protozoan parasite Ichthyophthirius multifiliis, which burrows into the fish’s skin and forms visible white cysts resembling grains of salt. Infected fish typically flash (rub against surfaces), clamp their fins, breathe rapidly, and become lethargic as the infestation progresses.

The ich lifecycle is temperature-dependent, which is why heat treatment is effective. At 25°C, the parasite’s lifecycle takes approximately 7-10 days. Raising the aquarium temperature to 28-30°C (for tropical species that tolerate this) accelerates the lifecycle and forces the free-swimming stage, which is the only stage vulnerable to medication. Combined with a UK-approved treatment such as eSHa Exit, Waterlife Protozin, or NT Labs Anti-White Spot, this approach eliminates the parasite within 10-14 days.

Prevention centres on quarantining all new fish for at least 2-4 weeks before adding them to an established tank. Ich is almost always introduced through new fish, plants, or equipment that has been in contact with infected water. Maintaining stable water temperatures and avoiding sudden changes also reduces susceptibility, as temperature fluctuations stress fish and weaken their immune response. For water quality management, see our aquarium water testing guide.

What Are the Fin Rot: Bacterial and Fungal Causes?

Fin rot is a bacterial infection that progressively destroys fin tissue, starting at the edges and working inward. Early signs include white or opaque edges on the fins, mild fraying, and slight redness at the base. As it advances, fins become ragged, develop dark or bloody streaks, and may erode to the body in severe cases. Both gram-negative bacteria (Aeromonas, Pseudomonas) and Columnaris bacteria can cause fin rot.

The primary cause is poor water quality. Elevated ammonia, nitrite, or nitrate levels stress fish and compromise their immune system, allowing opportunistic bacteria to take hold. In UK aquariums, fin rot most commonly appears in tanks that are overstocked, under-filtered, or where water changes have been neglected. Addressing the water quality issue is essential; medication alone will not resolve fin rot if the underlying cause persists.

UK-available treatments include eSHa 2000 (broad-spectrum antibacterial and antifungal), Waterlife Myxazin, and NT Labs Anti-Bacterial. Dose according to manufacturer instructions and complete the full treatment course even if symptoms improve. During treatment, increase water changes to 25% every other day to maintain water quality and remove dead tissue. Severely damaged fins can regenerate fully in healthy fish once the infection is cleared. Our water changes guide covers proper technique.

What Is the Velvet Disease (Oodinium)?

Velvet disease is caused by the dinoflagellate parasite Oodinium pilularis (freshwater) or Amyloodinium ocellatum (marine). It appears as a fine, gold or rust-coloured dust coating on the fish’s body, most visible when light reflects off the fish at an angle. Because the spots are much smaller than ich, velvet is often missed until it has progressed significantly, making it one of the more dangerous common diseases.

Symptoms include clamped fins, rapid gill movement, lethargy, loss of appetite, and rubbing against surfaces. In advanced cases, the skin may peel, and fish may gasp at the surface due to gill damage. Velvet is highly contagious and can devastate an entire tank population within days if untreated. Immediate isolation of affected fish and treatment of the entire display tank is recommended.

Treatment involves dimming or turning off aquarium lights (the parasite is partially photosynthetic), raising the temperature slightly, and using a copper-based medication. In the UK, eSHa Exit, Waterlife Cuprazin, or NT Labs Anti-Parasite are effective options. Copper treatments are not safe for invertebrates, so remove any shrimp or snails before dosing. The full treatment typically takes 10-14 days. Our fish health monitoring guide helps you spot symptoms early.

What Are the Dropsy, Swim Bladder, and Internal Infections?

Dropsy is not a disease itself but a symptom of internal organ failure, usually caused by bacterial infection of the kidneys. The hallmark sign is severe abdominal swelling with scales protruding outward like a pinecone when viewed from above. By the time dropsy is visible, the internal damage is often extensive, and the prognosis is generally poor. Early intervention with antibacterial treatment and Epsom salt baths may help in mild cases.

Swim bladder disorder affects a fish’s ability to control buoyancy, causing it to float at the surface, sink to the bottom, or swim at abnormal angles. Causes include overfeeding (especially dry food that expands in the stomach), constipation, bacterial infection, and physical injury. For constipation-related swim bladder issues, fasting for 2-3 days followed by feeding blanched, deshelled peas often resolves the problem. Bacterial causes require antibacterial medication.

Internal parasites such as tapeworms, roundworms, and hexamita can cause weight loss, white stringy faeces, and lethargy. UK-available treatments include eSHa gdex (hexamita and intestinal flagellates), Waterlife Octozin, and NT Labs Anti-Internal Bacteria. Deworming new fish during quarantine is good practice, particularly for wild-caught or pond-raised specimens. Maintaining excellent water quality remains the foundation of disease prevention. See our aquarium maintenance schedule for routine care.

What Is the Disease Prevention: The UK Fishkeeper’s Best Defence?

Prevention is vastly more effective than treatment in fishkeeping. The four pillars of disease prevention are water quality management, quarantine protocols, appropriate stocking levels, and stress reduction. Maintaining ammonia and nitrite at 0 ppm and nitrate below 40 ppm through regular water changes and adequate filtration keeps fish immune systems functioning optimally. Use our water testing guide to monitor parameters weekly.

Every new fish should be quarantined in a separate tank for 2-4 weeks before introduction to the main aquarium. This isolation period allows you to observe for signs of disease, treat any issues without medicating the entire display tank, and allow the fish to recover from the stress of transport. A basic quarantine setup requires only a small tank (20-40 litres), a sponge filter, a heater, and a hiding spot.

Avoid overstocking, which concentrates waste, increases aggression, and spreads disease rapidly. Follow the general guideline of 1 cm of adult fish per 2 litres of water for small community species, with adjustments for species that produce more waste. Feed a varied, high-quality diet in appropriate quantities, as overfeeding is a leading cause of water quality problems. Our fish feeding schedule guide and beginner fish species guide provide detailed stocking and feeding advice.