I have a simple answer that worked for me - turbulence.
Horses do not mind a turbulent water trough.
Some love it - they will  even make the "Wharrgarbl" sound effect like a dog
playing in a sprinkler.
A few may be spooked by it at first, some horses are such wusses about any
change at all, you know which of your horses are like this.

Bees avoid such excessive turbulence, so they will look for the alternate
water source.
Think lily pads, mosquito fish, and such.  Calm, contemplative ponds where
one can reflect on the reflecting light.

So, slap a solar panel on the barn roof, hook up a pump that jets the water
right across the surface of the trough, and use a float valve to maintain
the level of the trough so that the jet of water has the desired effect.
The pump need not run after dark, so this is a great no-battery solar
application.

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