Generally, it's thought best not to combine turtles with fish. Turtles don't require the same water quality as fish, which is definitely a good thing, considering the amount of waste turtles produce. The filtration and water change regimen for a tank combining these would be major, in my opinion, and enough to make me not want to try it. Also, a 30 gallon aquarium, especially if it's not full, as it wouldn't be for a turtle, doesn't hold much water, which causes problems with waste and temperature regulation for fish -- the turtle wouldn't want to have a heater, as he warms up when basking, and cools off in the water. Most fish need consistent temps to stay healthy, which would necessitate a heater. As for what turtles, I don't know of any, but I'm sure some turtle experts will chime in here!