I use 2 capfuls of listerine, it flavors the water for picky horses when you travel, and also kills mosquito larvae and keeps the bucket scum down. My water buckets is 27 gallon. (55 gallon drum cut in half)
I've got a 50 Gal trough and use a swimming-pool skimmer to keep the hay, leaves and pine needles out.
I drain and wash it every 3 days. In hot, sunny weather, I have to do it almost daily in order to keep algee from growing.
Bleach, iodine, or anything else that will kill algee, will also kill-off the "good" bacteria in your horse's guts..... NOT a good idea.