Fish get sick fast in saltwater tanks. Early action saves lives. Use this fish disease signs chart to spot problems before they spread.
How to use a fish disease signs chart
Start with a two-minute check, twice daily. Watch breathing, swimming, and appetite. Compare what you see to one clear symptom at a time.
Test water before you blame disease. Aim for salinity 1.025, temperature 78°F, and pH 8.1–8.4. Keep ammonia and nitrite at 0 ppm. Keep nitrate under 20 ppm for fish-only.
Many “disease” signs come from stress. New fish often hide and skip food for one day. Heavy breathing for hours is different. Flashing on rocks is also a red flag.
Log what you see with dates and photos. Note any new fish, coral, or food. Also note any heater or pump changes. This helps you spot triggers and patterns.
- Observe at feeding time and after lights-out with a dim flashlight.
- Check gill rate. Fast breathing plus hanging near flow is urgent.
- Test ammonia first if fish gasp at the surface.
For setup help, review your quarantine tank setup. For stability tips, see our reef tank water parameters guide.
This fish disease signs chart (symptoms, likely causes, first steps)
Use the chart below as a starting point. One sign can fit several issues. Combine signs with water tests and recent changes. When in doubt, isolate the fish in quarantine.
White dots like salt (0.5–1 mm), scratching, normal appetite early: Likely marine ich (Cryptocaryon). First steps: move fish to QT. Keep display fishless for 76 days. Treat QT with copper at 2.0–2.5 ppm (product-specific).
Fine gold dust, heavy breathing, rapid decline: Likely velvet (Amyloodinium). First steps: immediate QT. Add strong aeration. Start copper quickly. Consider a freshwater dip for 3–5 minutes if stable.
Thick white slime, peeling skin, clamped fins: Often Brooklynella, common on clownfish. First steps: QT and formalin-based treatment per label. Increase oxygen. Do not treat in the display tank.
Stringy white poop, weight loss, picky eating: Often internal parasites. First steps: feed medicated food with metronidazole. Use praziquantel for worms. Feed small portions twice daily for 10–14 days.
Frayed fins, red streaks, cloudy eyes: Often bacterial infection after stress. First steps: improve water quality. QT and use a broad antibiotic. Keep temperature stable at 77–78°F.
Black spots on tangs, mild flashing: Often black ich (turbellarian worms). First steps: QT and praziquantel. Repeat dose in 5–7 days. Improve nutrition with algae sheets daily.
- Fast breathing + hiding: check ammonia, then suspect velvet or gill flukes.
- Spots that come and go: ich life cycle is likely. Do not “wait it out.”
- Only one fish affected: think injury, bullying, or a new arrival issue.
Common mistake: treating the display tank with copper. Copper harms inverts and rock. It also makes future reefing harder. Treat fish in QT and keep the display stable.
Troubleshooting and prevention that actually works
Prevention beats emergency treatment every time. Quarantine all fish for 30 days minimum. Observe eating and breathing daily. Then treat proactively if signs appear.
Match your tools to the problem. Use copper for ich and velvet. Use praziquantel for flukes and black ich. Use antibiotics for wounds and fin rot. Avoid mixing meds unless directions allow it.
Reduce stressors in the display. Keep temperature swings under 1°F per day. Keep salinity changes under 0.001 per day. Provide hiding spots and steady flow. Feed a varied diet.
Plan a simple response kit. Keep a spare heater, air pump, and ammonia badge. Keep a 10–20 gallon QT ready. If a fish stops eating for 48 hours, act fast.
- QT size: 10 gallons for small fish, 20+ gallons for tangs and angels.
- Water change rule: 25% if ammonia hits 0.25 ppm in QT.
- Feeding: 2 small meals daily. Add vitamins 3 times weekly.
For a deeper dive on parasites, read our marine ich treatment walkthrough. It includes timelines and fallow planning.
Sources: Colorni & Burgess, “Cryptocaryon irritans Brown 1951, the cause of ‘white spot disease’ in marine fish”; Noga, “Fish Disease: Diagnosis and Treatment”; Hemdal, “Marine Fish Health & Feeding Handbook”.
Use this chart as a fast filter, not a final diagnosis. Confirm with water tests and careful observation. When you act early in quarantine, you protect every fish in the tank.
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