I have new tank - about 5 weeks. Fish for last two weeks. 56 gallon. Well planted. Currently in the brown diatoms phase but otherwise healthy (if young) as far as I can tell. .25 or less ppm ammonia (but slightly non zero), no nitrites, 5ppm nitrate. Slightly higher ph than ideal for tetras (7.6). Tested using API master. I have 8 Cardinals (for now), 2 otos and 5 peppered corys. There are also a (few) snails from the plants In the last day, first one, now two cardinal tetras have rapidly (a few hours) developed a faded area. One side obly, perhaps a third of the fish. No visible damage to the skin, it isn't raised or anything like ick, just faded. The affected fish swim slowly and sometimes hang out either at bottom or top - typical stress behavior. Ideas?