Sorry for your frog's death This may sound like a silly question... But did you mix the reptivite and water and try to feed that to the frog by itself as a sort of supplement? I've heard of calcium overdoses causing seizures in frogs before. I'm probably misreading that though :P

Hm... This is almost definitely not the cause for the frog's death, but you may want to consider removing the snail for both its sake and the remaining frogs. Snails and frogs both secrete mucous and toxins from their skin that could be irritating. They also absorb chemicals through the skin very easily. Even if they live together in the wild, they wouldn't be as likely to come into contact with these toxins on a regular basis (thinking of a story where someone was having problems partly due to housing a giant land snail in a tank with a frog.)

Also, can you post a picture of your tank? It sounds like it'd be interesting looking.