Common Freshwater Fish Diseases: Symptoms, Causes, and Treatment

How to Tell If Your Fish Is Sick

Fish can’t tell you they don’t feel well. But they do show it. Before diagnosing a specific disease, look for these general warning signs that something is off:

• Hiding more than usual
• Clamped fins (held tight against the body instead of spread open)
• Gasping at the surface
• Loss of appetite
• Unusual swimming pattern (tilting, spiraling, floating upside down)
• Visible spots, patches, or growths on the body
• Rapid gill movement
• Rubbing or flashing against objects

Any of these behaviors is a signal to test your water first. Many “disease” symptoms are actually water quality problems. If parameters look good, then move on to diagnosing a specific illness.

The Most Common Freshwater Fish Diseases

Ich (White Spot Disease)

What it looks like: Small white dots covering the body and fins, like the fish was sprinkled with salt. The fish will often flash (rub against objects) and may clamp its fins.

Cause: A parasitic protozoan called Ichthyophthirius multifiliis. It’s the most common fish disease and spreads rapidly. Usually triggered by stress, temperature drops, or introducing infected new fish.

Treatment: Raise the temperature to 86F (if your fish can tolerate it) to speed up the parasite life cycle, and treat with an ich medication containing formalin, malachite green, or copper. Treat the whole tank – ich is in the water, not just on the fish. Continue treatment for at least 2 weeks after the last spot disappears.

Fin Rot

What it looks like: Ragged, fraying, or disintegrating fins. The edges may look white, brown, or black. In severe cases the fins can rot down to the body.

Cause: Bacterial infection, almost always secondary to stress or injury. Poor water quality, fin nipping from tankmates, or physical damage from sharp decorations creates the opening for bacteria to establish.

Treatment: Fix the underlying cause first. Do water changes to improve quality and remove the stressor. Treat with an antibacterial medication like Seachem Kanaplex or API Fin and Body Cure. Fins will regrow if caught early enough.

Velvet (Gold Dust Disease)

What it looks like: A fine gold or rust-colored dust on the skin, most visible on the head and near the gills. Fish will flash and scratch, and may breathe rapidly. Easier to see under a flashlight held at an angle.

Cause: A parasitic dinoflagellate (Oodinium). More dangerous than ich and moves faster. Affects gills directly, which is why fish often show respiratory distress early.

Treatment: Dim the tank lights (the parasite uses photosynthesis). Treat with a copper-based medication. Temperature increase helps. Treat for a full 2 weeks minimum – velvet has a long life cycle and stopping early leads to reinfection.

Dropsy

What it looks like: Severe bloating of the body, with scales that stick out like a pinecone (called “pineconing”). Often accompanied by a curved spine, protruding eyes, and pale gills.

Cause: Not a single disease – dropsy is a symptom of organ failure, usually kidney failure caused by bacterial infection. By the time pineconing is visible, the fish is in serious condition.

Treatment: Isolate the fish in a hospital tank. Treatment with antibiotics (Seachem Kanaplex in food works well) can help if caught early. Add aquarium salt at 1 tablespoon per gallon to help the fish regulate fluids. Prognosis is poor once full pineconing develops, but early treatment gives the best chance.

Swim Bladder Disorder

What it looks like: The fish floats at the surface, sinks to the bottom, swims sideways, or has trouble maintaining its position in the water. Most common in fancy goldfish and betta fish.

Cause: Can be bacterial, constipation, overfeeding, injury, or anatomical in fancy breeds. Bettas are especially prone after eating dry pellets that expand in the stomach.

Treatment: Fast the fish for 2-3 days. Feed a thawed frozen pea (squeeze the outer skin off) which acts as a laxative. Raise the temperature slightly. If bacterial, treat with antibiotics. Reduce the water level so the fish doesn’t have to work as hard to reach the surface.

Setting Up a Hospital Tank

A hospital tank is a separate, bare-bottom tank you use to isolate and treat sick fish. Treating in the main tank means medicating your filter bacteria, your plants, and your substrate – all of which can cause problems. A 10-gallon tank with a simple sponge filter and heater is all you need. Set it up before you need it.

Prevention Is Easier Than Treatment

• Quarantine new fish for 2-4 weeks before adding them to your display tank
• Test your water weekly – most disease outbreaks follow a water quality problem
• Don’t overstock – crowded tanks stress fish and spread disease faster
• Feed the right amount – uneaten food decays and spikes ammonia
• Watch your fish daily – the sooner you catch something, the easier it is to treat

Not Sure What’s Wrong? Let AI Help.

Diagnosing fish diseases from a description is hard. Diagnosing from a photo is much better. Tank Wiki’s AI photo diagnosis feature lets you upload a photo of your fish and get a likely diagnosis along with suggested next steps – without having to scroll through forum posts hoping someone recognizes the same symptoms.

You can also log every treatment in Tank Wiki’s medication log – what you used, the dosage, the start and end date, and whether it worked. That history becomes invaluable the next time something goes wrong, or if you ever need to consult a vet.