Fish get sick. Sometimes due to water quality problems, sometimes from pathogens introduced with new fish, sometimes from stress. Knowing how to identify signs and act quickly can be the difference between losing a fish or saving it.
Before anything: most diseases in aquariums are caused or aggravated by poor water quality. When a fish gets sick, the first thing to do is test ammonia, nitrite, nitrate and pH. Fixing the water solves many problems before you need medications.
General signs of problems
Watch for behavior changes: fish that stopped eating, that stays still at bottom or surface, that isolates from group, that swims erratically, that rubs against decorations.
Physical signs: spots, dots, color change, damaged or closed fins, swelling, bulging eyes, gasping breathing.
Most common diseases
Small white spots all over body, as if fish was sprinkled with salt. Fish rubs against surfaces trying to relieve itching. May become gasping.
Parasite Ichthyophthirius multifiliis. Very common, especially after stress (transport, sudden temperature changes).
Gradually increase temperature to 28-30°C (accelerates parasite cycle). Use specific ich medication. Treat entire aquarium, not just affected fish. Continue treatment for at least 3 days after spots disappear.
Fins with whitish edges, torn, fraying. In advanced cases, fin may be almost completely consumed.
Bacterial infection, usually opportunistic. Almost always associated with poor water quality or stress.
Improve water quality immediately. Frequent partial changes. In mild cases, clean water resolves. In more serious cases, specific antibacterials. Aquarium salt can help in some cases.
Cottony white or grayish patches on body or fins. Looks like cotton stuck on fish.
Fungi that attack already damaged tissues (wounds, lost scales) or immunocompromised fish.
Specific antifungals (methylene blue is an option). Improve water quality. Identify and treat the original cause of tissue damage.
Golden dust or rust appearance on body, especially visible under angled light. Fish rubs, may have rapid breathing.
Dinoflagellate parasite. Very contagious and can be fatal if not treated.
Darken the aquarium (parasite uses photosynthesis). Increase temperature. Use specific medication (copper sulfate in some cases, but be careful with invertebrates). Treat entire aquarium.
Severe abdominal swelling, raised scales (pinecone appearance seen from above). Eyes may be bulging.
Not a disease itself, but symptom of organ failure (usually kidneys). Can be caused by internal bacterial infection, parasites, or organ failure from other reasons.
Prognosis usually poor when scales are already raised. Can try antibiotics in bath or food, epsom salt to reduce swelling. Many cases are irrecoverable.
One or both eyes protuberant, bulging out of orbit.
May be physical trauma (hit), bacterial infection, or symptom of systemic problem (like dropsy).
If only one eye, probably trauma. Improve water and observe. If both eyes, likely infection or internal problem. Antibacterials may help if it's infection.
Quarantine
Every new fish should go through quarantine before going to the main aquarium. A simple 20-40 liter aquarium with filter and heater, where the fish stays 2-4 weeks. This allows observing if it's healthy before risking contaminating your entire aquarium.
Most people don't do this. And most disease introductions in aquariums come from new fish.
Using medications
Some general rules:
- Identify the problem before medicating. Wrong treatment is useless or harmful.
- Remove activated carbon from filter during treatment (it absorbs medication).
- Follow exact dosage. More is not better.
- Complete treatment even if fish seems cured. Stopping early allows resistant pathogens to survive.
- Many medications are toxic to invertebrates (shrimp, snails). If you have them, treat in separate aquarium.
- After treatment, do partial changes to remove medication residues.
Aquarium salt
Aquarium salt (not table salt, which has iodine and anti-caking agents) is a mild treatment for various conditions. Helps with external parasites, mild infections, and stress.
Typical dosage: 1-3 teaspoons per 20 liters, depending on fish tolerance. Some fish (corydoras, plecos, tetras) are sensitive to salt. Others (mollies, guppies) tolerate well.
Salt doesn't evaporate, only leaves with water changes. Don't add more salt with each change, or concentration will keep rising.
Prevention
The best way to deal with diseases is not to have diseases:
- Keep water quality impeccable
- Don't overstock the aquarium
- Feed adequately (not too much or too little)
- Quarantine new fish
- Minimize stress (sudden changes, aggression from other fish)
- Observe your fish daily to detect problems early