For that use, Purigen will be your best bet. Just be careful not to use anything but prime or sodium thiosulfate in the tank, and to recharge the purigen diligently.knobhill;1458272;1458272 said:Thanks Wyldfya! It's not as if I am neglecting the tank, I change the water weekly. Even with a 100gpd RODI unit, it takes a long time to produce enough water for more frequent water changes on a 175g aquarium. The chemical media is simply a supplement to the filtration.