I took a water sample in to my parasitology lab, centrifuged it down and analyzed it under a microscope. I found several flagellated protozoa: brown/green ovoid, 5 micronsX3 microns with small flagella on pole. My professor said they were common, non-parasitic aquatic life common in ponds. He then said that the favored treatment was very, very dilute formalin. Has anyone every used formalin for controlling protozoa? Any other good treatments? I have done 50% water changes at least once a week and the water is still just not quite clear.