I was using Wal marts great value distilled water for top off and water changes. However, I started having massive green hair algae problems. My tank started having low hits for phosphates with the massive growth and I couldnt figure out where it was coming from. I droped to feeding my fish to once every 2 to 3 days to no avail. I finally had the distilled water tested and bingo. Thats where it was coming from. It wasnt much, but continually adding it to the tank caused the phosphate to become a huge problem. I just bought a RO/DI unit and have already seen a huge diffrence. I vacume up the algae out of the sand and it hasnt grown back. Now I just need to get a better cleanup crew to finish off the algae on the rocks. So beware. I dont know if they get thier DS water from diffrent places but here in South Alabama there are phosphates in it.