To reiterate some of the points brought up earlier:
- Musk turtles are weak swimmers. It needs shallow water, and needs to be able to reach the surface for air by crawling on structure, not having to swim.
- It must have proper UVB lighting. I recommend a Reptisun 5.0 bulb.
- Water must be heated to appropriate temperature.
- A warm basking light and basking area must be provided.
- A cycled tank would be nice, but very difficult to achieve without a very large enclosure. Turtles are messy. Focus on mechanical filtration and regular water changes.
- If you want to keep small fish with it, go right ahead. The turtle will enjoy eating them.
- Overfeeding turtles can lead to uneven shell growth. Avoid the temptation to feed it every time it begs for food.
Turtles have very little in common with fish. I think this point has been well made already, but you really need to study up un them. Musk turtles can be engaging, long lived pets with proper care.
FYI, I have a snapping turtle that I hatched from the egg in 1986.