this is new one what is that black shadow is it his belly? or what
heres KOI he is a right show off
OK my replacement boy is still not a very happy bunny, he has had something to eat though. My original "Koi" is very similar but I think the replacement is more orangey.
It is pretty common to see stomach contents and working organs through the flesh of light-bodied fish. If it turns purple, or spreads, it could be a batercial prob/septecimia...but likely its just the normal absence of pigment that makes the black visible.