Plant Gro tells you to buy a test kit to monitor the chelated iron levels, and add more fertilizer when the levels are low. I use that fertilizer too, and I dose it once every week or two. It depends on my amazon sword leaves. If the new ones that come out are smaller than the old one, then I add more fertilizer. After the leaves grow to about the same size, I don't dose again until the leaves become smaller again. I see my amazon sword as a pretty good indication of the levels in my tank.