Take this for what is worth, some people obsess over watering their chickens. Granted I am not in any way saying they are wrong for doing that. My flock has 2 waterers, a 3gal inside the coop which I put iceblocks in on hot days and a pvc homemade gravity one in the run which I fill with clean water every day. So that being said after a heavy rain storm,talking torrential here, the girls are out drinking out of the muddy puddles. Now I know that there is no way that this is the cleanest water that is available. Even though I tell them not to drink it they don't listen. I wouldn't get worried about a little algae. Not to mention my hens were wrestling a toad into pieces the other day,cant imagine that they are the cleanest critter either. Chickens do what chickens do!