Two days ago one of my guppys dies randomly then today I wake up to find that another one may be on the same path. I noticed him lying on the bottom upside down. when he tries to swim he uses only one fin and the other stays immobile. he also seems to be breathing rapidly. His tail also appears to be niped. Is he sick or injured by other fish??? his tank mates are three wild darters not even an inch long, a common pleco, another guppy, and three ghost shrimp. Please help