I have one that I’ve used in a stall (worked great) & now that I have the horses at home (I didn’t build stalls), I use it in their run-in shed (their side is 12 x 24’). I love it.
I suspect that if your horse is not already a known bucket pooper, he won’t poop in his water in a tub either. I’ve never had poop in mine, nor seen it at the last place I boarded, where all stalls had heated muck tubs in winter.
I dump mine very rarely – as was correctly noted above, horses do not require laboratory pure water. Water conservation also matters. I use old nets from work (I’m an aquatic biologist), similar to little aquarium nets, to scoop out debris. I might dump it when it’s low maybe 1-2 times per month. I just top it off daily (2 horses will usually drink it in 12-24 hrs) & the horses prefer that over the nearby unheated trough when it’s cold out. The only reason that trough doesn’t have a plug heater in it is because there’s no safe way to run power to it without digging a trench & I can’t be bothered (I have another trough that I do heat).