Help with very high nitrates


In my last thread, I posted that I lost all of my fish in the span of about 4 days (180g tank running for 13 years)....I have one Yellow Tang left. Over the past few weeks, I have changed 100% of my water with RODI (fresh filters), have vacuumed my sump of all detritus (also cleaned the gravel really well with my siphon while changing water), started dosing NoPox, have put no food into the tank, and my nitrates won't budge.

I have a huge protein skimmer that is skimming normally.

I don't know what else to it possible that all of my rock needs to be replaced?