that's why you only have 2 now, and at least one of them got sick, as well as not noting the difference between swimming turtles and walking turtles. Here's some helpful advice I heed you to take if you plan on keeping this turtle alive.
Walking turtles: Turtles that walk on the bottom (musk turtles included) are very poor swimmers that need shallow water and many ways to clamber around in their habitat. At least until the turtle is a little bigger, keep it by itself, because as you should be well aware, bluegill can be rather aggressive in captivity. Without the fish, the tank should be more hygenic, as well as being able to keep it in shallower water, which would be better for such a small turtle.
Swimming turtles: these would be like your other turtles (the painted and the map turtle). these can easily swim in deeper water, and they are much different in behavior than a turtle like a musk turtle.
The people on here do know what they are talking about. Please heed their advice. That's the end of my rant of the day.