I have six serpae tetras (among other fish) and one of them has developed black patches on his body. Basically, it looks like he's dirty. I've been trying to research to see if it's a disease and can't find anything. The only thing I found is "black spot", but it said "small black specs" and these are more like large discolorations. Is something wrong with him? He's acting fine, eating, and it's only on one fish...
TIA!