It can be a tricky combo to do and it will all depend on your rays and your frt. I've been keeping a mbu and a frt in my ray growout tank and all has been fine when I feed my frt quite well but at times for some reason the frt will chase the white spot on my pearl x marble but not any other ray that has been in there. The mbu and the turtle allways leave each other alone. I did try the frt in my main 2200 gallon tank and it was good until my one motoro decided it didn't like it and kept pouncing on it and biting it's flippers, the other 13 rays could care less. To me I would never recommend this combo unless you have alot of time to spend at home initially to figure out how to make this work. I currently have a pregnant scobina that has been in with the frt now for a couple years and should be giving birth so I'm going to leave the turtle in to see what happens as I can tell at least 6 hours before when a ray is going to give birth so I will sit up and watch the birth and see what happens. I've allready got a spare tank setup for the frt if I chicken out or need to move it during the birth. The pups will be moved right after birth anyways.