I thought that tank size was good, until you said you were going to put that much water in it. THis only leaves half the tank (20 gallons) for frogs. What kind of frogs were you looking to keep? I would suggest building a vivarium to cater to a specific breed, not making a vivarium and then trying to put frogs in it.
If you do want lots of water, I would suggest having fire belly toads. You can make a wonderful half land, half water setup. This is what I have for my toads.
