Several of the different fish in the 30long have been flashing or scratching themselves on the bottom or objects in the tank. The water quality is excellent and has been for a long time. I have done a few small water changes. I can see no visual cause such as ick or other disease symptoms. I even examined filter material, gravel, and the water with my 1200x microscope. I could not find anything I think would cause the fish to scratch. I strongly suspect that there is an undetected parasitic organism at the root of the problem. I am contemplating a Malachite/Formalin treatment. Before I do this, I thought I would ask for comments and suggestions.