You mentioned that if you put 3 frogs in a 75 gallon tank, you would never see them. I have three D. galactonotus in a heavily planted 135 gallon paludarium and they are almost always out and about and visible. The four D. leukomelas are in a 72 gallon bow tank with a stream, shallow pool with smooth pebbles, lots of plants and they are almost always out hunting and "playing" in the pool. All of my other frogs are also visible most of the time, albeit in somewhat smaller tanks, including the D. auratus that started out by being shy until I offered them even more hiding places, which made them feel secure enough to come out and hop around a lot more. So go ahead and use your larger tank for just a few frogs if you have the room. Some of the most fun is setting up a design with water features, being able to grow an enormous variety of beautiful tropical plants, as well.
It's a hobby you will certainly enjoy. Good luck!

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Patty
Lost River, Idaho
4 D. auratus blue
3 D. galactonotus pumpkin orange splash back
5 D. imitator
4 D. leukomelas
4 D. pumilio Bastimentos