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sometimes they flop on a leaf here and there then go back in the water and swim is this a good thing swimming too much?

Define "swimming too much"...

How long have you had them?

What's the water temp and basking area temp? Do they have proper lighting to bask under?

What do their shells look like? If they're healthy, your turtles could be basking when you're not around.

Unless said otherwise, from what you said, I'd say your turtles are fine. RES (if you have this species of turtle, the common kind on this site) are aquatic turtles, as their name suggests, spends most of their time in the water, their bodies and webbed legs help for swimming and diving. They can also hold their breaths underwater for a very long time, such as sleeping at the bottom of a body of water, to hibernating months on end at the bottom of a lake or whatever.

However, if you mean that they won't stop moving frantically (maybe bashing into the glass repeatedly?), then I would refer to the post above me for more insight, that is not normal. RES like to drift around in the water or swim in a slow relaxed pace, not in a fast and panic-like state. Refusing to surface and dry off may account for sickness or that the basking area is inadequate (such as light, UVB, and heat). A healthy turtle should bask in a warm and well lit area with UVB rays for several hours a day, at least 1 hour a day I'd say from my experience.