that looks very bad to me scale rot,scale rot in my opinion it's caused by bad water condition but also can be caused by infection I think but that all relates to bad water quality.
Since I got 700 liters aquarium with over filtration non of my fish's were ill,in fact they never been even if they fight and hurt them selfs, a guide to healthy fish's is a good diet and good water but also fish must have space to swim and feel safe in aquarium.