i began using purified water on one of my plants and to me it seems to only strengthen the plant. I doubt highly it will have a better result in yield but it might. i wanted to experiment myself and it seems my plants health began to increase greatly but im only on day 3 of flowering so i'll keep you posted if i finish flowering before you. I try to stick with distilled water or tap water, leaving it out for 24 hours to rid the chlorine and bring the water to room temperature as not to shock my plants! what i like best overall for water though is natural rain water. It's pH level is great for MJ... but its like the tap water, i leave it out for 24 hours so it hits room temperature. plus rain water is more on the acidic side of pH which MJ likes! plus its natural! you cant go wrong!