Map turtles are not "fully aquatic". They, like all N. American species, must be able to climb completely out of the water and absorb heat and UVB, and a lot of the time they will sleep out of the water. Some people build basking platforms that rest on top of the tank to maximize water volume (or utilize HOB filtration).
I have posted in a few similar threads, I have a larger (9") Jack Dempsey who has been living with my 4 turtles (2 red eared slider, 1 hiero river cooter, 1 florida red belly) for 4 years in a 125 gal tank. He is large enough that the turtles know they can't harm him and it works well, they even eat the same pellets as the staple of their diets.
Everyone above is right tho, turtles require a lot of filtration. If you REALLY want to mix turtles and fish, look into larger (at least 6" by maturity) species of fish and keep a close eye on everything. If you get them acclimated to living together early in their life and give them enough room, it's certainly a possibility. But no two animals will react the exact same way, so you just have to get kind of lucky!