FastAquatics
Step-by-step how-to

How to identify aquarium fish disease

Step-by-step diagnostic flowchart for fish disease. Visual symptoms, behavior, water-test indicators - matching symptoms to disease categories.

Beginner7 steps

What you'll need

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.

Related

Browse all how-to guides, calculators, setup guides.

Aquarium-keeping fundamentals

Whatever specific topic brought you here, four fundamentals govern long-term aquarium success: water quality, parameter stability, biological filtration, and species-appropriate husbandry. Skip any one and the others struggle to compensate.

Water quality: ammonia + nitrite at zero, nitrate under 30 ppm freshwater + 10 ppm reef. Test weekly with API or Salifert kits. Use our water parameter checker to score your readings against your tank type.

Parameter stability: stable wrong parameters beat fluctuating ideal parameters. Most fish tolerate a wide pH range if it's stable. Sudden swings of 0.4+ pH or 5+°F kill fish faster than chronic suboptimal values. Use temperature controllers (Inkbird) + automated dosing for consistency.

Biological filtration: the bacterial colony on your filter media + rock + substrate is the engine. Never replace all media at once. Use our filter turnover calculator to size correctly.

Species-appropriate husbandry: research adult size, territoriality, diet, and tankmate compatibility before purchase. Use our tank stocking calculator + compatibility guides.

Frequently asked questions

How long does an aquarium take to set up? 4-6 weeks for full cycling + first stocking. Use our cycle ETA calculator + how long does cycling take.

What's the best aquarium for beginners? 20-gallon long. Big enough for parameter stability, small enough for budget + space. See beginner picks.

How often should I do water changes? 25-30% weekly. See water change frequency Q&A + water change calculator.

Why does my fish keep dying? 5 leading causes: uncycled tank, wrong species pairings, no quarantine, undersized tank, neglected water-change schedule. See full diagnosis.

Related resources

Saltwater livestock · Freshwater livestock · Coral catalog · Care library · Q&A library (222) · Glossary (127) · Disease database (50) · Calculators (29) · Interactive tools (7) · Husbandry deep-dives · DIY projects · State legality directory