I've seen wild arum growing round native ponds and the fish dont seem bothered but obviously there is a large volume of water to dilute any toxins if there are any. Are you concerned that the fish might eat the berries? If so remove the flowering spike as the flower fades or as the berries set and start to form.

Otherwise only poisonous if large quantities of the foliage are allowed to ferment in a small pond--actually true of practically all plants.