Yeah that is a pretty small water area, only about 50 gallons. I think your right, boxies would do better in your setup. Only thing is, you may want to just stick to spotteds. I am assuming you are breeding them, and you dont want any added stress by different species of turtles being introduced. But if you must, a pair of Eastern Box turtles would work, depending on where you live.
Chris
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0.1 Red Eared Slider, 1.0 Common Snapper, 1.0 Bearded Dragon and a 55gal Native Fish Tank with a 6in LM Bass, 3 Crawdads and a Pleco.
I use to have a collection of Leopard Geckos that I bred, but have sold them.