I used to do this for my mare's trough, though I kept going out to the barn to find all her fish would be missing within a few days. I lived near the coast at that time and it wasn't unusual to find seagulls or herons snooping around, so I assumed they were the culprits.

Nope. It was my horse. I went out to the barn one morning and caught her staring intently into the water. She'd lower her muzzle just enough to dip her whiskers in the water to get the fishes' attention, then she'd slurp them up when they got too close. It's disconcerting when your horse makes unbreaking eye contact with you while grinding up poor, unsuspecting goldfish. She didn't get anymore fish after that.