A 20gal Long is minimum. But I would suggest a 40gal Long or Breeder. I personally think a 20gal is cramped.
Chris

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Hey whats up, I'm Chris and I currently have 0.1 Midland Painted turtle, 1.0 Western Painted turtle, 0.1 Red Eared Slider, 1.1 Leopard Geckos, 1.0 Bearded Dragon and a 55gal African Cichlid Tank