It's going mainly on the sides of the tank, but only in portions where the water moves slow. It's also goving on the power filters, the water moves slowly there too. I have been medicating for a different problem (after this started) with Maracyn and Maracyn II and it don't seem to be killing it. I just started noticing today that the fish seem to scratching more than usual. If I clean the tank it does come back in a couple of days.