It's best not to change all of it at once. I have four little bags of it in a reactor and I change the oldest bag each week. The "bag" is a ladies knee high stocking cut in half, 10 pair for $3. I swish the bags in RODI each week and tie a knot in the end for each week it's in the reactor...four knots gets tossed out and a new bag. This method changes 25% each week. I run it 24/7