I have asked many an expert about this and there is no definitive answer. I keep fancy goldfish and koi the same size in a 50 gallon tank. Some of my fancies have grown curled tails, some have not and none of the koi seem to have this malady. The two answers I have received are: 1. The fin curling is due to genetics, in that, it has never been purposely bred out of fancy goldfish. Goldfish breeders do not keep their fish long enough to see this nor do they care. 2. The other reason is that the nitrate, not nitrite, levels in your water may be too high. I checked my water and my nitrates were a little high but not really. This testing kit is expensive and not really worth the cost to anyone but a die hard fish person. So my answer is, nobody seems to know.