My NO3 and PO4 have been creeping up for some time now (see graphs on the left). My GFO is quite new, filter wool is replaced every 7-10 days, carbon dosed extensively, and Chaeto is growing well, although getting a bit covered by Cyano. My thinking was that detritus was accumulating somewhere. To make a long story short, my skimmer was Islowly getting clogged by stuff. After I gave it a good 20 min citric acid wash (let it work with citric acid), I washed it and put it back. Only then I noticed how clogged my skimmer was. I will update if the parameters will or will not creep back to their preferred values.