One of my neon green rasboras died whilst I was away for three days. I thought it might have something to do with the black spot on it’s stomach. Any ideas? Do I need to do anything for the other fish in the tank?
If it's possible that you didn't discover the fish for 48 hours or longer, I'd be inclined to suspect that the black spot is caused by food in the stomach or feces in the intestine starting to rot. It certainly appears that the skin in that area is also deteriorating. There is not a fish disease that I know of that manifests as a black spot on the belly.