This is a common issue, due to a slight imbalance in N and P coming in in the form of food vs going out in skimming, chaeto, off-gassing, etc. As others have said your 2 options are remove a little phosphate with GFO or add a little nitrate with a supplement.
Also just to point out that the relationship between N and P is analogous to that of Ca and alk. They are both used and added in relatively stable proportions, but since there is so much more Ca than alk (and so much more N than P) when in the proper balance, when you use or add both you see the changes more readily in the alk or P than in the Ca or N. For the same reasons, the tests we use are much more sensitive to small amounts and small changes in alk and P than they are for Ca or N. So it is common to hear people say that when they dose more (or remove more) of either set that they see the alk (or P) change much more than the Ca (or N), like you are seeing. If our tests for Ca or N were as sensitive and accurate as those for alk or P, we would see them change too, but only very slightly.
Also just to point out that the relationship between N and P is analogous to that of Ca and alk. They are both used and added in relatively stable proportions, but since there is so much more Ca than alk (and so much more N than P) when in the proper balance, when you use or add both you see the changes more readily in the alk or P than in the Ca or N. For the same reasons, the tests we use are much more sensitive to small amounts and small changes in alk and P than they are for Ca or N. So it is common to hear people say that when they dose more (or remove more) of either set that they see the alk (or P) change much more than the Ca (or N), like you are seeing. If our tests for Ca or N were as sensitive and accurate as those for alk or P, we would see them change too, but only very slightly.
