Okay, is this used liverock?
I just checked my journal, last year, on june 29 I had .26 ppm phosphates, running hi cap GFO AND rowaphos in 2 reactors, it took me exactly 1 month to get readings down to 0.00 however at that point, I noted that I had an algae outbreak. I stopped using the GFO and ROWA completely. On 15th of Aug my phosphate was back up to .21ppm. I restarted GFO and added the bio pellets. It dropped back down to ~.05 by Aug 28, and stayed at those levels until March of this year, when it finally started reading 0.00 all the time.
So my hypothesis is in my case, that a) the quick change in PO4 levels allowed a strain of algae to take off, and it was very likely because the PO4 and other contaminants from months of using tap water in the tank, were leeching out of the substrate and rock, and in fact it took several months to come out completely.
I figure almost everyone is using used liverock of some sort, even if it was dry. ??