I rinse out and refill their small indoor waterer just about every day. It's nice and clean. The hanging 8 gallon outdoor waterer however, tends to only get a rinse and refill every week. It gets a ton of green algae in it. I spent a good hour yesterday trying to scrub it out, but I could only do so much. It's too big and cumbersome to clean inside the house on a regular basis. I should probably replace it with a couple of smaller waterers that I can change and clean more easily. I often wonder if the green algae really harms them or not. I've had the huge waterer for about 5 years and no harm that I can see is coming to the chickens.