redearslider.com

Would it be safe to feed my RES some canned Tuna?

It wouldnt hurm him in a small amount and as a VERY RARE treat.......packed in water, not oil.

Tuna is generally used to entice a turtle that has not been eating for a prolonged period of time. With a food like that, there is always the risk that your turtle will decide to refuse anything BUT tuna so be careful if you're feeding it as a treat.