I use RO because I don't like declorinating the water with chemicals. I also think that my tap water in general has a little too much stuff in it. I have had just about every algae issue you can name in the past, but I don't think it has anything to do with the water. In my case, it had to do with lack of regular ferts, CO2, high light, and too long of a photo period. Now that I have addressed all of these issues, my algae is minimal and what I would consider normal levels.