golden zebra loach

Hi, I have 2 juvenile golden zebra loaches, 3 guppies, 2 shrimp, and 1 nerite snail...checked my water parameters last night and everything was fine..0 nitrates, pH 6.8, temp 74, and now this morning one of the zebra loaches has a red sore on his side with some rapid breathing, I got this loach...