I majored in fisheries and aquaculture in college, there are so many bad things in stagnant and even running water that no matter what you do for treatment, you will not solve the problem. Aquatic insects, crustaceans, birds, manure, runoff, etc. all contribute to the concentration of pathogens, microorganisms, parasites, toxins, and pollutants especially in stagnant water. E. coli, giarrdia, dysentary, crypto, norovirus, just to name a few. I would exclude it immedietly. Your local health dept. will ussually test water samples for free and tell you all of the nasties that are present in a sample.