First is the whites trying to eat the cagemate. Whites will not hesitate to eat anything that moves, even if it is close to the same size. Much like horned frogs. Second is cross toxicity. Many amphibians can be toxic to each other without actually being "poisonous." Stress, of course, is one of the largest factors in a mixed tank. More animals=more competition for food/space=more stress. Yet another issue is transmission of disease and parasites. More animals means more waste. More waste means more bacteria. Frogs can carry some nasty loads of parasites in them as well. If the stress builds up and the parasites take a good hold of the frog, death is definitely possible. The animals aren't from the same area either. Maybe if they were brozen frogs and leopard frogs, or something that naturally would occur in the exact same small woodland vernal pool. I would seperate them ASAP though.
Mixed tanks are just a bad idea. They simply have no benifits. If you choose to put more than one animal in a tank, make sure the tank is VERY large, the animals are the same size, and that they are similar species (such as something like gray and green tree frogs). And I probably would not even do that.
But that is my opinion. It is your tank, your animals, and your money. So if you really want to, go for it and best of luck to you.
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Andy Maddox
AIM: specktroutfishin
MSN: Poloboy32486@hotmail.com
Houston Herp Key
The Reptizone