too much mechanical or chemical filtration can strip your water of essential elements. this, combined with very efficient fuge and or skimmer can leave you with water that is basically 'too clean'.. If this is the case and you don't do regular water changes, you are just compounding your problem by not replentishing those elements. you want water free of phos, heavy metals, silica, etc. thats why we use RO. to take out the bad stuff. a trace of nitrates IMO helps deepen some corals colors. so in a nut shell, yes, you want water that is not too bad(extra phos, nitrate, etc) and not too good (stripped of all nutrients,cal,mag,stront,alk,etc,) hope that helps