I had two dalmation mollies die yesterday. My ammonia/nitrites/nitrate numbers were good, but it was time to do a water change to manage the nitrates anyways. They started out by resting on the bottom, listless, then slowly deteriorated until death. My creamsicle molly is starting to rest on the bottom. He did respond to food today and ate. I noticed he has some white growth on his tail. I didn't notice this on the dalmations, but maybe I missed it because of the coloration of the fish. What does my fish have, and how do I fix it??