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Bigger is better for a turtle tank. You will need decorations for the turtle to rest on at the top of the water as well as a basking area with uv light and heat bulb. I would go with musk turtles for a beginner turtle. Also make sure your filter is too big for the tank as turtles are very messy.
I keep pleco with my turtles and depending on the type of turtle and size of the tank you can have a few fast fish like danios but not much else