Step-by-step
Step 1: Test water parameters firstMany "diseases" are actually water quality. Test ammonia + nitrite + nitrate immediately. Detectable ammonia/nitrite = stop here, fix water first. Most "sick" fish recover when water is fixed.
Step 2: Photograph symptomsTake 3-5 close-up photos in good lighting. Easier to compare to reference images than describing what you see.
Step 3: Identify visible patternsWhite salt-grain spots = ich. Gold/bronze dust + rapid breathing = velvet (saltwater). Cottony patches = fungus or columnaris. Saddle-shape on back = columnaris. Bulging eye = popeye. Pinecone scales = dropsy.
Step 4: Watch behaviorScratching/flashing = parasite. Gasping at surface = ammonia or low oxygen. Lethargy = many causes. Hiding + fast breathing = stress or disease. Lying on side = critical.
Step 5: Use the diagnostic toolOpen
/tools/diagnose/. Pick tank type + closest visible symptom. Returns probable diagnosis + treatment + alternative diagnoses.
Step 6: Cross-reference disease databaseBrowse
/diseases/ for the suspected condition. Each entry has detailed symptoms + treatment + prevention.
Step 7: Treat in quarantineMove infected fish to a hospital/QT tank. Most medications kill biological filter, coral, or invertebrates. NEVER treat in the display tank with shrimp, snails, or coral.
Frequently asked questions
How do I know if my fish is sick?
Behavioral signs first (lethargy, hiding, not eating, gasping). Then visual (spots, fungus, fin damage). Test water - many "sick" fish are actually suffering from water-quality issues.
What are the most common aquarium fish diseases?
Marine: ich, velvet, fin rot. Freshwater: ich, columnaris, fungus, popeye, swim bladder, dropsy. Most are stress-triggered + treatable in QT.
Can I treat fish without quarantining?
Some medications are reef-safe at the prophylactic level (ParaGuard) but most kill coral + inverts. Always treat in QT for any aggressive medication.