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I keep small turtles like yours in a 20 gallon long with just a small inside floss filter. I feed them every other day in another container. I give them more than they will eat and give them at least 4 hours or so to eat. Then I wash them off and put them back into the 20 gal long tank. That way you don't have to clean out the larger tank very often.
I feed them chopped small fish, worms and pelleted food.