Another skimmer cup full of light skimmate today, this one not quite so foul. Hanna Checker reading 0.00, so all the PO4 that was in the water column has been taken up by the bacteria and has either been skimmed out or is clinging to the surfaces of the tank and rock. I'm going to stop the dosing and let the skimmer run in the rock tank for another day or 2, then do a water change and wait a few days before checking again. Fingers crossed!