I haven't had a horse do that, but I did have one of our beef heifers do it. She broke THREE 150-gal water tanks by stepping in and out. And the water was constantly disgusting.
I did not find it amusing at all. It downright pi**ed me off. Especially since she was doing this during the worst drought in 50-years, so constantly dumping out nasty water was a huge waste.
When she broke the third tank, she earned herself a trailer ride and came home in little white packages.
OP, I also encourage you to raise the tank to discourage the behavior. It gets old fast when you have poopy water and broken tanks. I could not raise my tank because if I did, the calves could not reach the water...