Thanks for your replies, guys! I have removed him from the tank and he is in clean cool water. I will do daily water changes. Also will try that remedy!!
I'm considering putting him in the fridge, is that a good idea? My apartment is on the warmer side. Or I could keep him in my room with the A/C on if that would suffice.
His hand fell off and there's just a few bits of flesh hanging now, but overall looks more improved from a few days ago. His head has some weird spotties on it now and his legs are on the redder side and really veiny. Can't tell if he's just stressed in his new temporary house or if it's something else.