I've seen a lot of people doing this... putting goldfish in their horses/cattle water troughs to eat away algae and all the nasty crap. Any opinions on this? I'm not finding much online about it, and I'd like to know if it's 100% safe for horses. My water troughs are always so filled with algae and it sucks ass to clean it every couple days.

Unless you have a 200+ gallon tank with a filtration system, I wouldn't do it. Rather put a capful of apple cider vinegar in/move the trough to the shade so it's harder for algar to grow, and also dump and rinse it daily. (Seems like a pain but it's far easier than scrubbing. And only fill it halfway if it's big enough so you're not dumping hundreds of gallons out)

Fish are hard because it's not a great environment for them, they really need filters and water changes. Plus, they're easy prey for birds and horses (yes, horses can and will eat them). They also produce ammonia, which isn't good for the horses either.

I clean my troughs weekly with apple cider vinegar, about 2 cups for a hundred gallon tank, scrub it well and refill. mosquitoes will still lay eggs in the water weekly, they hatch quickly and I'd rather my mares not be drinking them.

Catfish clean better than goldfish, but need more space

Goldfish aren't good in water troughs. They are a cold water fish, and when the trough is outside the water gets to warm for them, also for just 1 goldfish you need a 50 gallon trough to start, and as they get larger you need more room for them (They can get up to a foot long), which means for one gold fish even if you start with a small one for them to live a healthy happy life your trough has to be about 500+ gallons for just one fish, and they really don't eat algee. They are also a very messy fish, and you need a good filtration in the trough for them to survive. Channel cats are better for that, but you need around a 300 gal trough for just one channel cat since they get very big.

Everyone hit the nail on the head. Goldfish are the filthiest fish out there. They produce obscene amounts of waste, and the water quality would be atrocious with them in there. Plus, a diet of algae alone would not be sustainable. Keeping fish in water troughs to begin with is just dumb honestly, it's better just to just scrub out the trough bi-weekly in my opinion