Replace sand filter and motor with a new quad de filter

I would agree, sand is probably best for commercial pools.

DE is workable too, and delivers outstanding water quality . . . but the typical commercial DE equipment demands more skill than most pool staff can provide.

My very *first* large commercial pool customer had a below water level open *vacuum* DE filter. It was a beast -- when it malfunctioned, it would drain the pool, or at least 150,000 gallons of it!