Personally, I consider it cruel to the fish, and actually, the waste produced by the fish would pollute the water for your horses. Fish produce waste in the form of ammonia which eventually converts to nitrate- combine that with sunlight and you have the perfect recipe for algae.
For either fish, expecting them to survive off the insects, and algae produced in the trough would not be a nutritionally balanced diet, and the poor water quality would stunt their growth or kill them. Many "sucker fish" or algae eaters are tropical (require a heater) and living without leaves them prone to disease. Both fish are known for producing a lot of waste, which is why they need filtration to be healthy in aquariums or man-made ponds.
I've cared for fish and hoofstock. At least once each week pour a splash of bleach into the trough, scrub it out, dump it, rinse it well, and refill.