Finally came to the realization that he needed to be euthanized before his suffering was drawn out any longer. I came home tonight and filled a little throwaway tupperware with good tequila; I figured I should show him at least some respect. I almost lost my nerve when I put my net in his tank to pull him out and had to wait for a second. I nearly changed my mind until he flopped right into the net like he knew what was happening. He went quickly and peacefully.
He was never a healthy fish, but I really hope I kept him as comfortable as I could the 8 months he struggled through with me.