Hi, I have a school of 6 diamond tetras. The tank has been cycled, ammonia/nitrite at 0, nitrates at 10ppm. 5 of them are normal, healthy, active. However, one of them changed colors today. He became very dark, swims stiffly (as though his body won't bend), seems weak (drifts with the current a lot), and generally seems unhealthy. These fish are fairly new (about 2 weeks), there is a good chance that this one was just too stressed or even injured from the start (I was not very pleased with the idiot clerk at the LFS. He seemed to know very little about fish, and even less about catching them. He used his hands to take the fish out at one point, and I told him to put it back and get me another one.) It is very likely that this one will be dead by morning, but if it isn't, does anyone have any ideas on what might be wrong with it and how I can treat it? The store has a guarantee, but I'm pretty sure if I go back it'll be the same idiot clerk working...like he is every day. I've already had to replace one of them because he actually left it in a bag WITHOUT WATER for about 30 seconds without noticing.