Is there a risk of drowning with tree frogs if you make a pool deep enough for the fish? I would not recommend putting the fish in there. Also, my gray tree frogs are quite aggressive eaters, I can imagine one taking a snap at the fish and then struggling to get back out of the water. What size tank do you have? I have 4 gray tree frogs in a 35 gallon tank, and there wouldn't be enough space in there, in my opinion, to give the guppies the water they need and space for the gray tree frogs.