Frogs and toads have toxins in their skin that is harmful to each other. The secretions of a toad will kill a dog very fast if untreated. Leopard Frogs don't have very many species that can attain a size that they will not eat or be eaten by the other frog. Wood Frogs will not get eaten or eat leopards if they are both adults. The problem is that wood frogs are more like an American toad. They don't live by the water like other frogs do. I've found some in the woods decently far from a pond. And the size of your tank is a bit small to keep both of them. 2 leopards will do good in a minimum 29-30 Gallon long. A wood frog doesn't need as much space since it's more terrestrial then aquatic but, you would need a 40-55 or more to do this. It looks more natural and decreases territorial standoffs. I have a 75 Gallon Vivarium with American Toads (a sexed pair) and Northern Leopard Frogs (sexed also), this is the minimum for 4 of them even with lots of hides and plants. I don't discourage you but I would say to see if your frog is happy. Why I got sexed pairs is because it to me feels more natural and the 2 species are happy. With a 20 gallon your one frog is probably happy, they aren't usually social with each other. So they don't need a friend. But a mate imo is something that everything and everyone wants so if you get a bigger tank, you'll find that would be all you'd need.
You can checkout my Viv I have info posted on there about stuff that May interest you
http://www.frogforum.net/showthread.php?t=34251
Don't try this unless you have years of experience and knowledge about both species.
Don't add a tree frog because the leopard frog will eat it. Mine can jump higher than 3 feet that I know of.
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