I have zero hope that this fish will survive, I've had it for a little over 2 years now and in the past couple months it got extremely skinny then ended up going back to normal size and a few weeks later ended up with a tumor type bump. One side of the fish was a larger bump than the other side, but had no problems with eating or swimming or anything so I let it go. This morning I got up and the tumor looks like it exploded inside the fish. There's no blood or anything coming out of the fish, just looks like it's bleeding internally inside.
I'll post some pictures just because I'm curious what has happened to it. It's a zebra danio, I only have 3 zebra danios in the tank, no other fish and there hasn't been anything other fish for very close to a year now. Please don't judge me on the algae in the tank, I've been having issues with it for quite some time now, it will go away and come back in a few weeks. The only way I could get semi-clear photos is when it stopped swimming and sat on the bottom, otherwise the fish can and will swim almost entirely normally.
It is actually much redder than in the photo.