Actually, to me it looks like it's having a fit. Perhaps it's got a problem on the inside [nerve issues/muscle spasms]. Like Steve said, perhaps leave them with only enough water to keep them moist, because if it is a muscle spasm[or other ailment], then they could potentially drown.

You should take them to be checked out by a Vet, but in the mean time make sure that they can get to the surface to breath and if not, do as suggested and keep only enough water to keep them moist.

Do they do this on a regular?
Also, it could potentially have something to do with their food, so you should monitor wether or not this happens after they eat or after they do something different from normal.

Take into consideration how they were before they started to do this, what's changed and how frequently they do it now, and maybe you can back-track and stop whatever it is from causing them to do this.

But in the end, I still recommend a Vet.