Do you have an area where the turtle can haul out and bask? Musk turtles are more aquatic than some but still require a place to dry out and UV light. Also they are pretty carnivorous so the tetras will most likely be picked off eventually. I have my EPT in a 75 filled about 5/8ths up with a big pile of drift wood for it to climb out of the water onto. There is not enough room in my tank for a GT.