Hi all, I woke up and 3 off my turquoise rainbow fish had turned more or less completely black. No change in behaviour and still eating. Are they getting ready to mate? All other fish are fine, water tested well. Just wanted to know if this was a thing they do? I've only had them around 3 months and they're rescue fish. Just for comparison I've added 2 pics, one black and the other the usual colour.