Ken, where is the crud located- top, top and sides down to a typical standing water level or is it on the bottom too? Do the tanks have large corner cutouts? Scrape some crud off and put it in a container with some bleach. Does the crud turn color or disappear? If not, it is doubtful that the crud will disappear with a bleach shock and some other form of cleaning may be required.   Rudy Briney Bug- Panama City, Fl