Reef fish can look fine until they suddenly crash. Most disease outbreaks start with small, easy-to-miss changes. Learning early reef fish disease signs helps you act fast and save livestock.
Early behavior changes you should never ignore
Behavior shifts are often the first clue. Watch your fish during feeding and after lights out. A healthy fish explores and responds to food. A sick fish often hides, drifts, or stays in one corner.
Breathing rate is a key alarm. Count gill beats for 15 seconds, then multiply by four. Many reef fish rest near 60 to 100 beats per minute. Rapid breathing plus surface hovering suggests low oxygen, gill damage, or parasites.
Flashing is another common sign. The fish darts and rubs on rock or sand. This can point to flukes, ich, velvet, or irritation from ammonia. Check your reef tank water parameters before you medicate.
Appetite changes matter even more than spots. A fish that refuses food for 24 to 48 hours is telling you something. Some species are shy at first, so compare to its normal routine. Use a log to track eating and breathing daily.
- Feed at the same times each day to spot missed meals fast.
- Watch gill movement before the room lights turn on.
- Note hiding, headstanding, or swimming into flow repeatedly.
Physical signs on skin, fins, and eyes
White spots can mean ich, but size and pattern matter. Ich spots look like salt grains and come and go. Velvet often looks like dust or a matte coating. Velvet also causes very fast breathing and sudden deaths.
Look for frayed fins and cloudy patches. Fin rot often follows stress, fighting, or poor water quality. Red streaks in fins can suggest bacterial infection or ammonia burn. Check ammonia and nitrite at 0 ppm, every time.
Eyes give strong clues. Cloudy eyes can come from injury or infection. Popeye can follow trauma or gas issues. If one eye is affected, suspect injury. If both eyes swell, suspect water quality or systemic infection.
Also watch the belly and waste. A pinched belly suggests internal parasites or starvation. White, stringy poop often points to gut issues. In that case, isolate the fish and plan a feeding and treatment strategy.
- Use a flashlight at an angle to spot velvet “dust” on fins.
- Photograph the fish daily to compare spots and fin edges.
- Check for excess slime coat and clamped fins after stress events.
First response plan: stabilize, isolate, then treat
Start by stabilizing the tank. Raise dissolved oxygen with surface agitation and clean socks. Confirm temperature at 77 to 79°F and salinity at 1.025 to 1.026. Keep pH stable, near 8.1 to 8.4, without big swings.
Next, isolate the affected fish if you can. A simple hospital tank works. Use 10 to 20 gallons for small fish, and 30 gallons for larger tangs. Add a heater, lid, sponge filter, and PVC elbows for shelter.
Do not treat the display with copper or antibiotics. Reef inverts and rock absorb many meds. Instead, confirm the likely cause. For suspected velvet or ich, plan copper in quarantine with a test kit. For flukes, praziquantel is common in quarantine.
Here is a practical example. A new wrasse breathes fast and hides on day three. You also see light flashing. Test ammonia and find 0.25 ppm in the display. You do a 25% water change and add aeration. If breathing stays high, you move it to quarantine and begin diagnosis. Use your quarantine tank setup checklist to avoid delays.
- Keep a seeded sponge filter ready in the sump at all times.
- Match temperature and salinity during transfers to reduce shock.
- Do not mix medications unless you know interactions and doses.
Many “disease” cases are stress plus water issues. Test first, then treat with a plan. If you need a step-by-step flow, see our marine ich treatment guide for quarantine timelines.
Sources: Noga, E.J. “Fish Disease: Diagnosis and Treatment” (2nd ed.); Colorni, A. and Burgess, P. work on Cryptocaryon irritans; Hemdal, J.F. “Diseases of Marine Fishes.”
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