Thank you, everyone, for your input.
Unfortunately, I lost him last night.
I'm completely crushed.
He had one white eye and open wounds. I don't know the reasoning for this, but my turtle may have accidentally hurt him. I've seen it before. (Though, she is not aggressive at all and would never do it intentionally)
*I don't think it was the water quality. I cleaned my two filters and everything in the tank recently. My water is very clear, oxygenated, and the surface is constantly moving.
Thank you so much for trying to help. I appreciate it.
It's hard to take care of fish. I think all of ya'll agree. Good luck with your pets, everyone. Please allow them to live full, happy lives.
Rest in Peace, Cucumber. You will be very much missed.