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I have a RES who is under a year old. I have yet to figure out the sex of it but decided to name it Kevin in the meantime. I have a very large tank for it and the habitat is perfect. A guy I work with was wanting to get rid of his painted turtle. Would it be ok if I put a painted turtle with my RES without knowing the sex of either? From what I understand, the painted is probably younger too. I have more than enough space for both in the tank. Im just worried about territorial issues, fighting, and that kind of stuff. Any thoughts?

How big is your tank?

I think its a 40 gal. tank

If they are of comparable size, it should be OK. But be aware that RES are usually more competitive than painteds and will likely outcompete it for food. Once they get larger, if Kevin is indeed a male then you will likely have to separate them.