There is little relationship between GH/KH and PO4. GH is a measure of the combination of Magnesium (Mg++) and Calcium (Ca++) ions in the water, while KH is a measure of the Carbonate (CO3–) and Bi-Carbonate (HCO3-). While the Phosphate ion (PO4—) is famous (or infamous) for combining with metal ions and precipitating them out of the water, it is unlikely that PO4 would combine with Carbonate/Bicaobonate, and if by some miracle it did, this would serve to lower the KH by preciptating these species out of solution. The same would go for Mg/Ca. Phosphate is a highly reactive substance and thats why it is highly prized by plants and animals alike. Phosphate is so important that it is normally the limiting substance in an ecological system. Phosphate pollution in the environment is a serious issue because it changes the balance of species in a system. Those species which are better adapted to uptaking PO4 excel while those less capable soon falter and disappear.