You have a
FO tank, right? With bio-balls I would be surprised if your
nitrates were 0 but in a
FO tank with a reasonable bio-load you should have no trouble keeping them at a manageable level. I like to do a 10% water change each week but I really don't think that water quality is your problem. Why don't you pull your clown out and put her in a hospital tank until you are sure she is OK?
