I don't own any fish yet but I REALLY want to use a Kritter Keeper so I thought I'd give my theory about what might be going on because if I'm right then I can go ahead and get a Kritter Keeper! =]
I think it's the tank itself and I don't think cleaning it will help if you've done it several times before.
I don't know anything about arcrylic but I know that waterbottles leach chemicals into the water when they get hot. It might be leaching chemicals. =/
Plastic is made using petrolium products which probably aren't safe for fish....
I'd get another Kritter Keeper after Shiro-chan is completely better and put him in that and put the heater in the exact same spot. If he gets sick again let him heal again and move his heater. Try sandwhiching it between a 2 layers of gavel. the problem might be the constant contact with a heat source on your tank.
After all... my experience with any kind of plastic has been hot+plastic BAAAAAAD!
So once you've cleaned your tank and moved the heater change the water a few times even though a fish isn't in there. (I'd do this while the fish is healing if shirochan got sick again from the new Kritter Keeper) Then when Shiro's healed again you can put him in the tank when the heater is sandwhiched between 2 layers of gravel.
If he doesn't get sick again then the tanks constant direct contact with the heater was the problem.
If he gets sick again then it might have been the plant or the cave and I'd just replace them.=P
And of course if he gets sick yet again I'd just ditch Kritter Keepers all-together. =[
I hope this helps but you don't have to take it because once again: I don't own a fish.... so I could be COMPLETELY wrong. =/
I REALLY hope shiro-chan gets better!!! He's just too cute to go to the rice paddies in the sky! =[