I was never actually sure what Peanut Butter had. It might have been columnaris, but not definitively. There are some older posts of mine up with regards to that. He did eventually recover, although we're not completely sure why or how.
Anyway, since I have two siphons, maybe I'll vinegar one and boil the other and see which one survives. That one will be the winner.
The pH in the tanks has dropped to about 6.2. I've done a partial water change (using a measuring cup. Talk about inefficient!) and I'm keeping a watchful eye on things.
Thanks for all your help! I'll let you know what happens!