I'm not sure when the rocks stopped leaching, but I do take very meticulous logs of my parameters so I can provide some insights in the PO4 over a long period of time. Around March 2020 PO4 was hovering around 0.8 (there were also THICK mats of GHA everywhere), and at this time the only nutrient export I had was my skimmer. I started planning the ATS (which I think I finished around the end of April), and started carbon dosing, which had minimal effect (I think NO3 might've been limiting). Over the next 3 months I wrestled PO4 down to 0.3 - 0.4 by aggressively running seachem phosguard (the ATS was providing negligible growth at this point, a combination of the initial seeding time required as well as the lighting schedule not being optimal). I then heard about LC so I started dosing that instead of phosguard, and was able to get PO4 in a range of 0.05 - 0.15. Until March 2021 I didn't change anything - Skimmer, carbon dosing, ATS, LC, and my phosphates were in the same stable range (although eventually I started dosing LC only the few days after changing the ATS). Then i started the purge treatment so I turned off my skimmer and reduced carbon dosing -- PO4 initially shot up to 1.1 and I don't want to talk about the rest. 3 months later (June 2021) I turned my skimmer back on and since then I've maintained PO4 0.03-0.07 using just the skimmer and ATS.