You could try a non-chemical product called Viresco Aqua, which we use in our three ponds to keep them completely clear of string algae. It was originally developed to promote fish health and they discovered that it also eliminated algae. It's sold online (from the UK, but prices include worldwide postage). Two of our ponds contain fish (koi and goldfish) and all three are used by various pets and wildlife for drinking water. If you have no fish it's likely that a small dose would keep yours clear for a year or more. Check them out at www.viresco-uk.com/information/about_viresco.asp
