What's the difference between RO and DI water? Is one better than the other? I am currently using tap water, and my green hair algae is out of control. I do have PUR unit that I am not using so, I was wondering about that. But DI water is something like $1.00 a gallon in my area, I think. Is there a cheaper souce for DI water? I only need 1o gallons a month. So what would be most cost effective?