Please a diagnosis someone!

[craigj]

My 4 inch blue-spotted grouper has dramatically lightened in color and has two small brown spots on his body. He also shakes from time to time. As of now, he is breathing normally and eating normally, and I have not seen him scratch his body. The water conditions are good. He is not being pestered by any other fish. I think what might have happend was I changed the salinity too fast. Could that do it? Its a 75 gallon tank and I poured about a gallon of RO water in it. Would that change the conditions drastically enough to stress him out? Please somebody try to diagnose this and tell me what I can do to help him, if anything. Thank you.

 

[craigj]

Ok, sorry. The nitrites and ammo are both at 0. Nitrates are 20ppm and pH is 8.3. The water temp is 80 and the s.g. is at 1.022. The only recent activity is the addition of a snowflake eel. I suppose he could have brought in the disease. My water levels have been stable for months. Any more information needed let me know. Any help would be greatly appreciated.