I have a mare that I raised. All her life she's liked to play in the water trough occasionally. No big deal. But now not only is she playing in it. She's also taught the 2 year old to play in it. I think it's became their game. Since it got hot, I'm having to fill my 250 gallon water tank twice a day now. They'll even drag it out in the pasture some. Had to have a concrete pad poured last week because they kept it ankle deep in mud. I can't come up with a way to stop them. We have a wood fence it sits under so I tried removing the bottom board and only putting it half way through. That didn't work. I watched her still put her feet in it splashing. Helpppppppp my water bill is desperate. 

What about buying a kiddie pool? It probably won't last long but they are cheap. I have a gelding like that and that is what I am about to try. 

Put in a automatic waterer 

Cut a "lid" out of plywood and leave only enough room for them to reach in and drink.  We do this up north to help block the wind so that our tank heaters can keep up with the -20--30 wind chills, but would also work in your situation. If you're worried about a sharp edge you can line it with a pool noodle.

This only works if the trough is large enough and the horse doesn't paw or chew on the auto water. We have one that does it and a small trough she can still completely empty and then break the auto water. The larger ones she has broke the auto water part but the trough she couldn't move. SO ANNOYING. 

That's what I was about to say. We've had 2 and they break them. 

I thought about that and looked at them. But I was afraid if the bottom broke they would get it hung on a shoe or leg. 

Get a larger water trough, like a 1000 gal. or a concrete trough..

I would get one of those larger ones then the can get all the way in. I have a mare that has demolished one of those Rubbermaid troughs. I thought about a bigger trough so she can just get in and go got a swim. My trainer has a small round Rubbermaid trough which I am not sure the can get in and doubtful they can paw in. This mare gets her babies all involved. 

where can I find the one in the 3rd pic? 

I have one that does this too.  We had a basic metal stand around the house that was the perfect size to put the tub on.  It was tall enough that he couldn't get his leg in it but short enough that he could drink from.  Cured my problem.

Check your Tractor Supply Store, if they dont have them you can order threw them, the one in the 3rd picture thats a galvanized tank that sells here in Seguin at the D&D farm and ranch store. Theres some really good stock tanks out there Tarter and CountyLine are really good brands, Google them and see if you have any around your area. You can get the larger ones in a 1000 or a smaller size in around 660 gallons I think thats the gallons rate but its somethink like that. I like the plastic ones better, they last way longer then the galvanized. 

I put Rubbermaid tank up on 8" cement blocks so my mare couldn't get her front legs in.

I feel your pain. I've tried EVERYTHING. Mine are still smarter than me and reason how to destroy tanks. I've tried raising them. I've strapped them to the fence. The only thing I've found to work is hanging regular buckets up high enough that they can't get a foot in. I can't even begin to tell everything I've tried because it makes me look insane. I have three that are terrible part hippo tank crawling fools.