so i bought some emperor tetras. ive lost 2 in about 24 hours. i noticed before they died their faces turned bone-white and their gills turned really red. The white doesn't appear to be external either, like a fungus. It looks like they're turning white. i checked my ammonia it was at 0. i did a 50% change anyways.... i have 3 left and the color on one of them appears to be washing out. any ideas on what is going with them?