Fin Rot and Fungus in Aquarium Fish: Signs and Prevention

Fin rot and fungus are two of the most common diseases in aquarium fish. Neither usually appears out of nowhere; both tend to follow dirty water, injuries or ongoing stress. Spotting them early and fixing the tank conditions is the key to recovery.

Recognizing fin rot

Fin rot is usually a bacterial infection that takes hold when a fish is weak or its fins are damaged. Signs include:

• Frayed, ragged fins and tail that get shorter over time.
• Fin edges that turn milky white, red or blackened.
• Red fin bases or blood streaks; in severe cases it can spread to the body.
• Clamped fins, sluggish swimming and poor appetite.

Note: fins torn by nipping tankmates or sharp decorations usually have clean edges and heal if the water is clean. With fin rot, the edges erode gradually, change color and spread.

Recognizing fungus

Water molds usually attach to skin or fins that are already damaged, or to unfertilized eggs. Signs include:

• White or gray cotton-like tufts on the body, fins or mouth.
• White patches that gradually spread and may turn greenish gray as debris sticks to them.
• Rubbing against objects and lethargic swimming.

Some bacterial diseases also cause white patches around the mouth that look like fungus. If you're unsure, ask someone qualified before using any product.

Common causes

1. Dirty water: high ammonia or nitrite from infrequent water changes, overfeeding, poor filtration or overcrowding.
2. Injuries: fin nipping, sharp rocks, rough nets.
3. Stress: fluctuating temperatures, incompatible tankmates, a new tank that hasn't stabilized.
4. New fish bringing in disease when added straight to the main tank without quarantine.

What to do when you notice it

• Test the water right away: ammonia, nitrite, nitrate, pH and temperature, if you have a kit.
• Do a partial water change with dechlorinated water at the same temperature. Vacuum debris from the bottom and remove leftover food.
• Isolate the sick fish in a separate tank with a filter and aeration, to monitor it and prevent spread.
• Find and remove the cause: separate fin-nippers, remove sharp decorations, reduce stocking.
• Medication: many mild cases improve with clean water alone. If antifungal or antibacterial treatment is needed, use it only as directed by your veterinarian and read the product label carefully. Don't mix several medications; many harm shrimp, snails and the beneficial bacteria in your filter.

Tracking recovery

Once the water is clean and the cause removed, fin rot usually stops spreading within a few days. New fin growth may be clear, thin and softer than the old fin; that's a good sign, not disease. Regrowth can take several weeks, longer for long-finned fish such as bettas. Meanwhile, keep the water stable, feed moderately with a varied diet and avoid startling the fish. Note the fins' condition every few days, or take photos to compare, because day-to-day changes are hard to see by eye.

Long-finned fish such as bettas, fancy goldfish and long-tailed guppies are more prone to fin rot because their fins are heavy, tear easily and snag on things. For these fish, clean water and a gentle current are especially important.

Prevention

• Change water regularly and clean the filter properly; never rinse filter media in tap water.
• Feed just enough and leave no leftovers.
• Don't overstock, and keep long-finned fish with peaceful tankmates.
• Quarantine new fish for a few weeks before adding them.
• Use smooth decorations and soft mesh nets.

When to see a vet

Ask a veterinarian experienced with fish when:

• Fins keep getting shorter, or the rot reaches the fin base or body even though the water is clean.
• You see red ulcers, raised scales, bulging eyes or a swollen belly.
• The fish stops eating, sits on the bottom, or gasps at the surface.
• Several fish in the tank are affected, or fish die one after another.