I agree with doughboy. I had hair algae problems for a while, both using RO/ DI water or tap. The macros grow like weeds!! Added a sump with caulerpa, and all the hair has been gone for some time, no returns, even after switching to tap water. I am somewhat lucky, though, because the tap water here in Columbus is low in phosphates, etc. I had it tested and also got the go ahead from someone who does research with the Columbus Zoo... JMO