Actually, leaving water out in the sun may promote algae growth. The algae comes from stuff floating around in the air that gets in the water while you are filling the container. I have a 275 gallon LDPE tote on a trailer that I keep for wash water or other needs. I use groundwater from an aquifer about 75 ft. deep and this water is sterile when it is pumped from the well. Unless I disinfect the water in the tote, algae will start to grow and the water will turn green. Once disinfected, this doesn't happen, since the tote is capped and nothing gets in. I keep my irrigation water in blue LDPE drums that are opaque. I don't mind algae in my irrigation water, but it plugs the drippers.