When a fish is seen scratching or rubbing on rocks or other objects (even the substrate), it is a sign that the fish's skin is irritated and the fish is trying to remove the irritating material. The material is normally cysts from either Cryptocaryon (marine white spot disease, ich) or amyloodiniosis ( Oodinium , marine velvet, or coral reef fish disease).
Velvet is generally smaller than the ich trophant. some signs of velvet are:
- Reduced or complete loss of appetite.
- Heavy breathing, scratching, flashing, head twitching, erratic swimming behavior (unfortunately velvet shares all these same symptoms with ich & gill flukes.)
- Swimming into the flow of a water pump/wavemaker/powerhead (unique to velvet).
- Acting reclusive (velvet causes fish to be sensitive to light).
My guess is Oodinum. Marine Oodinium (Amyloodinium) is present in a free-swimming and infective form in most ocean environments that wild fish are imported from. The Amyloodinium Dinoflagellate is extremely hardy and can withstand a wide variety of salinity (specific gravity) and temperature fluctuations.