eurithromyacin

[MitchM]

I think you need something else in there to take up the extra pollutants that could be leaching out from the sand and/or rocks. Lighter feeding and heavier skimming won't do it, IMO.
I've never tried Phosban, but a lot of people recommend it. Have you any macroalgaes growing in the system?
I'm at the point of adding some macros to my tank pretty soon to hopefully acclomplish the same thing.
Sand beds and "dirty" live rock (as in LR that doesn't have sufficient water flow around it) are phosphate sponges in themselves.

Mitch