First of all, what type of fish? That makes a huge difference.

Second of all, know that you're going to need some kind of hella filtration system to make this work. The FBTs have some kind of toxic secretions from glands in their skin. That is going to pollute the water like crazy. SO not only are you now managing your Ammonia, Nitrite, and Nitrate levels, you've also got chemical X in the mix.

All in All I'd say it would be safest not to try it out. I would worry for your fish more than your frogs for sure.

However, if you have something large, let's say a 50 gal or so, and it's got a riparium set up, where the frogs have plenty of moist on-land options and the fish have a seperate water-area, that could actually work out. You'd probably be able to do water changes before Chemical X built up to lethal amounts. But once again, I'm not sure if there are very many people out there who know the effects of FBT toxin on fish.