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i've seen my RES do this a few times...when he's out basking, once in a while i'll see him twitch...sometimes it's his arm or leg, a few times even his head. i'm concerned because i have an older RES and i've never seen her do this! is there something wrong with him?

nope. Mine do that all the time. Maybe an itch or something. Nothing to worry about. I'm actually surprised that you've never seen the other turt do it.

I've never seen mine do, but I can also never get close enough.

I can relate to that, only one of mine will actually bask in front of me, but if I get too close, it will jump in the water, I ask them: " Why are scared?, you already know me. )

My RES will do that when he's been basking for a while. It usually means he's getting too warm and will either adjust his position to get further from the basking light or go back into the water.