I use ro in my reef tank (get it from lfs) and well water for my 90 and 150. I only battle some beige turf-type algae that was introduced off of some liverock I bought. I do have occasional cyno, but it is not usually too bad and have taken care of that with chemi-clean when I get tired of looking at it. My reef tank (50 gal) started off with well water, but kept having hair and cyno issues, now I unfortunately introduced bryopsis algae, but I don't think that has anything to do with water quality (hitch-hiked in on a coral).
