My betta, Ariel, has this ??? i dont even know.. but this tiny part of the back of his tail is gone.. i 1st noticed it when he was upset about us getting him a snail. i quickly dismissed it as a temper tantrum. The snail has been gone for a while and the little part may possibly getting worse (slowly)
since the picture isnt the greatest quality, i'll describe it then post a pic
Ariel is a vibrant red with very flowy fins. The part in question is void and the edges of the void is a darker red possibly even black. The edges of the rest of his fins are translucent. I sure hope it was simply a temper tantrum and not some sort of disease.