A few of my bloodfin tetras have been coming down with some type of sickness. Their face seems to be degenerating, like it's melting or something. now it's eye looks like it's falling off. I don't know what's wrong, my pH and everything is fine, and my other fish are doing fine, but just the tetras one by one are falling to this. I already had to put down one tetra and now I isolated another one, and I don't want to have to put down another one, but it looks like it's in so much pain.