Hi: All... We use "The Geo method" to treat the grey/black tanks. After dumping add a solution of Calgon water softener, dishwasher detergent, and extra bleach if necessary. Seems to keep the scum buildup to a min. and keeps the tanks slippery and fresh. All basic laundry products.
Do a google search of the geo method for the recipe!!! Alf
escape artist N.S. of Lake Erie