I agree they are pretty hardy little guys. I had one in with 3 fancy goldfish and it didn't have issues with tank water - water was good but I'm sure directly after feeding, had I done testing right at that moment, there may have been a little spike in amonia.
Anyway, if you pick up a new one, look for a "fat" guy. If stomach is sunken, or one looks unusually tiny compared to the rest, pass on that one. Also, as "cute and fuzzy" as frogs appear, there is no frog on this earth that won't eat other frogs if one can fit in it's mouth so try to get one of comparable size - your other one just may have been the victim of a homicide
