keep in mind, although some Po is found naturally in the rock, the rest has been put there by us and then is bound to the rock and sand.
do you flush out the back chambers at water change and how deep is your sand can you vaccume it?
Although a CUC may eat it it is still in the tank and can only be removed mechanically or in a well balanced biosphere.
In my case I had fish that would not eat well, so mama flooded the tank with food and the skimmer could not keep up, and the design of the cube exacerbated the problem.
Once I figured that out, I only did the WC from the Back of the cube and what came out was nearly skimmate. My sand unfortunately was too deep to vac.
To fix the prob I did a tank transfer to a 30 cube and simply rinsed the sand and rock. For the problem rocks w algae I used a little peroxide solution.
Using A HOB refugium with higher lighting and increasing the number of corals and the intensity of the tank lighting, I used the corals, macros and the cyano itself to pull the Po from the sand and rock. I treated the cyano like Chato so to speak, letting it grow and then siphoning it out. I did not use GFO or other binders. I did use a $60
amazon canister filter as additional mechanical filtration to help when I blew cyano and alhae of the rock and cleaned it weekly. About one gallon of saltwater.
As Randy mentioned, another method of removing Po bound on rock from dirty tank is to put it in a bucket or bin with GFO. It keeps it live too. Thats also kind of how I tested to find the bad rock. I put it in a bucket with fresh SW and flow, then tested the Po and No.
Muriatic acid will kill the rock it and is a common method to start fresh and dry.
Were I you I would consider a deep clean and rebuild. This would would remove the sources and allow the gfo to bind the Po and remove it from the system. My transfer to 4 or 5 hours and the rehab in the new tank took about a month and a half.
