i dont know how good it works but i heard that a little vinager in the bucket keeps the algae gone and is obviously harmless. dont know how good it works or how much to use but maybe somebody else has actually used this method and can inform us all.
I been pouring a little apple cider vinegar in mine the past 6 months... it works great, no more green. I just dump a little in their 5 gallons buckets when I rinse and refill them... probably 2 or 3 tablespoons worth.