It is very important that your horses have clean water to drink. Do not put fish in your horse's water tank thinking it will keep the water clean. Fish poop in the water and fill the water with bacteria and parasites and this is what your horse is drinking.
Mosquitos and other insects lay eggs in water, bird droppings land in the water and then your horse consumes the water that contains larvae and bacteria and gets parasites or diarrhea.
The tanks or automatic waterers need to be cleaned on a regular basis. Don't let fungus grow in them. Your horse consumes the fungal spores putting stress on the immune system.
Sand and dirt accumulate in the water. It may seem as if the sand settles to the bottom so your horse is okay to drink it. If the water is stirred around which some horses do as they drink, the sand is consumed and could accumulate causing sand colic.
Take the extra time to clean your horse's water tank or automatic waterers. Clean water is life giving. Keep your horses happy and healthy.