For the most part, African clawed frogs can be maintained at any water depth.The water should be standing or slowly moving. Water that is too shallow can increase stress levels while deeper water is enjoyed by males because their calls propagate better. There is no optimum water level. It has been my experience that minimum depths should be at least 1.5 times the length of the frog. 29 inches should not be a problem. You already recognized that these frogs are predators and will eat smaller fish, I predict a loss of a few fish. Good luck!