will he eat the fish... the turtle???

How big is the aquarium? what are the dimensions? what kind of turtle?

yes. it doesnt really matter what type of turtle exactly, how big the tank is, or what type of fish you put in there, assume whatever goes in with a turtle is turtle food. sometime sits not right away, but in general iot will become food eventually. there are things you can do to help the fish's chances, but in general dont worry about the fish too much if its in the turtle's tank.

There are more variables than that. Most turtles will eventually nip at fish in a small container but in larger tanks or ponds, there are some species which will not bother fish. The size of the fish and turtle as well as their species and tank size all factor into it. Because turtles make a mess eating and produce a lot of waste, I don't suggest keeping turtles with other animals in a tank situation (ponds are a different story). A turtle tank should be catered towards turtles and a fish tank towards fish. The water depths, filtration, etc. often conflict.