I actually used RO & UV filtered water from the grocery store machines and after 5 gallons I had the worst outbreak of purple slime. The filter must have been old and the 'sold out' light must have been broken... My PUR filtered water was MUCH better. I am back to using distilled water until I can buy a RO/DI filter. I just don't trust the PUR filtered water but for some reason I trust something someone else put in a gallon jug... It's strange because I tested some spring water once that I bought for a water change in a freshwater tank and one out of the three had crazy high nitrates. I really need to get a RO/DI filter with the inline TDS meter. Then I'll be sure of the quality of my water.
KG: I'll have to go back and check the test kit I used and make sure. Everything is fine and my algea is finally gone. The purple slime turned yellowish and died a few days after I switched to distilled water for top-off.