Yes some can who's do you have ? Some reactor media have been high Phosphate. The last was TLF Reborn but I'm pretty sure Julian fixed that. It was when it first came out. When reactor media is dissolved, due to low pH from CO2, the phosphate is released, just like Calcium, Magnesium, Alk, etc., and what ever else is bound in the coral skeletons / sea shells.
To your question, yes dirty reactor media that collects stuff and acts like a filter can but usually does not due to the low pH. It is mostly the phosphate chemically bound in the reactor media when you bought it. Allot of this reactor media is mined from old reefs that are now inland. It is not collected from beaches or the ocean bottom but inland pit mines. One could call it fossil coral, Julian does, TLF. And such can and often does, if it is collected in the upper regions of the formation, where it can be high in phosphate.
