It is very important to keep your horse's water trough clean. This is a small task that could be a huge factor in keeping your horse happy and healthy. It should be thoroughly washed everyday in order to remain free of bacteria and other substances that could be harmful to your horse's health.
When giving fresh water to your horse, it is extremely that you dump out the old water first, rather than just pouring new water on top of the old water. There will most likely be masses of bacteria in the old water- from either the horse's mouth or the environment, such as insects, feces, dirt, or old bits of food and hay, which could be detrimental to his health.
It is also important to dump and clean the bucket far away from the horse to avoid getting any bacteria on the hay he eats or in his living environment. Haul the trough away from his stall, and dump it in an area where the horse doesn't go. It's a good idea to also stay away from the water turnout as you don't want to mix the fresh water with the dirty water.
Use a scrubbing brush with a liquid soap and fresh water to thoroughly rub down the bucket. The brush should have strong, yet soft bristles to avoid scratching the bucket's surface. Scrub the inside and outside of the bucket, and make sure you don't ignore the groves in and around the edges. These are the places that algae and bacteria will hide the most.
Make sure you rinse all of the soap completely, again paying attention to the groves and corners, until you no longer see any soap bubbles. You may want to dry the bucket with a clean cloth before refilling the trough. Finally, fill the bucket with fresh drinking water, and then bring it back to the horse's stall.
It is a good idea to observe just how much water is left in the trough before dumping so you can get an idea of just how much your horse drinks each day, and how much you should be filling the bucket.
If at any point you notice tiny spots floating on the surface of the water in your horse's trough, dump and clean the water immediately. These specs could be a harmful bacteria, algae, or insect eggs that may hatch in the water. Anything that looks questionable should be dumped at once to keep your horse from consuming it.