i am using r/o water, but i usually top off with tap water, sometimes ill use distilled or ro to top off. there is quite a bit off flow in the tank, its a 20 gallon tank, and the filter does about 110 gallons an hr, with one power head doing 119 gph, and the other 110 gph. the tank dosnt get fed that much, the nitrites never go up past 20 ppm, (umm the last one in the cycle, i think its nitrites, ) and it gets at least 3 to 4 new gallons of fw a week, and water change every 2 weeks. all the corals are doing great, and so are all the inverts. a tang is out of the question, aside from the fact that its a 20, it is that sulfur bassed algae that burns the fish's mouth. i think im gonna go with some phosphor remover, and purigen, possibly som carbon for a bit just to really get rid of any exsess nutrients.