Thanks for your response. He did actually loose the arm last night. I've moved him into a smaller tank by himself and I think I'll keep him here for a few days just to watch him. The spot (where his arm was attached) was very red last night but today it's much better. And he's now swimming around and eating normally - he actually seems to be much happier here than in the other tank so that is leading me to believe one of his tankmates may be the guilty party...
Thanks again and I'll definitely check out your group. We've had our frogs just 4 months now, but they are so great and I've really gotten much more attached to them than I thought I would.