Hendrik, I think Callas are only dangerous to your fish if they actually eat the tubers. Just growing them in your pond should be fine for the fish. (since it's been a month, maybe you know for sure by now anyway?)
But I would just put a layer of rocks on top of the planting medium in the pot - so that the fish would have to work awfully hard to get at the tubers.
I do this for my lotus tubers, because lots of fish, and also ducks and other critters like to eat lotus tubers. Luckily, I have gravel driveways and walkways around the house so there are lots of clean rocks to be had.
