This is good advice, but I will say phosphorus absolutely can leach from rocks exposed to a high phosphorus environment. I see the effects of phosphorus in a large scale where I live in farm run off into lakes. the nitrogen restarts every year, but the PO4 accumulates and, while it can be bound temporarily, the only route to removal is through use in algae/plant growth. Any live rock exposed to decaying organics (aka dried out and reused) may contain large amounts of phosphorus... i also don't think his problem is PO4 though
Bacteria products are sham science. Send out a placebo product and it will do the same.
fixing you nitrogen cycle is the best thing to do. Truly, less food more water (i still like urchins, but Retro did give you the most sustainable answer)