Can you keep fish with a grey tree frog?

[Herpin Man]

If the environmental parameters are all appropriate- temps, humidity, etc- and the frog has lots of places to climb and be out of the water, then it should be workable. While many amphibians do have some degree of toxicity, it is generally only exuded under extreme duress. Tree frogs aren't just oozing poison all of the time.
I think one of the bigger challenges you have is figuring out how to keep the feeder insects from drowning.