Troubleshooting: Fish

Why are my fish scratching on rocks and sand?

A fish darts sideways and rubs its body or gills on rock or sand. An odd scratch now and then is normal; several times a minute, or several fish at once, is not.

Last checked 21 October 2026. How we check

What should I check first?

1. Count: how often, and how many fish?
2. Look at breathing. Fast breathing with scratching is urgent.
3. Look for spots or dust on the body and fins in good light.
4. Test ammonia, nitrite, salinity and temperature.
5. When was the last fish added, and was it quarantined?

What is most likely causing it?

Most likely first. The order is our reading of how often the sources link each cause to this symptom.

1. A parasite, often marine whitespot (ich) or velvet. Scratching plus spots, cloudy eyes, clamped fins or fast breathing. Most common within weeks of adding a fish that was not quarantined. Tangs are the classic carriers (LiveAquaria).
2. Flukes. Scratching and cloudy eyes without clear spots, often in tangs and angelfish (our reading).
3. Water quality. Ammonia or nitrite above zero, or salinity or temperature off. Usually affects several fish at once.
4. Normal behaviour. Rare scratches, eating well, breathing normally, no marks.

What should I do?

1. Fix any water problem first.
2. Watch closely twice a day for spots, breathing rate and feeding.
3. If you see fine dust like icing sugar and fast breathing, assume velvet and act the same day: see white spots on fish.
4. If a parasite is confirmed, plan to treat in a separate tank, not the display. Medicines that work on fish parasites kill invertebrates and can harm corals.

What not to do

• Adding a "reef safe" remedy to the display without knowing what it contains.
• Adding more fish until the problem is settled.

When should I get help?

Something dying right now? Go to the emergency playbooks.