answer:Continue your experiment. Next time use the filtered water, UV sterilized in a sealed (and sterilized) tank with pure oxygen, nitrogen, carbon dioxide or other pure gas (or no gas at all, if you can manage that). Eventually you can isolate the source of the algae-genesis. When I put plain tap water in plastic bottles that are no more than “reasonably clean” (just juice bottles rinsed with tap water), but have little or no air inside, I can store those for years with no algae formation. I think the spores are in the air, not the water. Of course, it may be the small amount of chlorine in my tap water that prevents the algae formation, but I hadn’t wondered about that until now.