Unique Marine Digest-It Boat Waste Water Holding Tank Treatment 16 Concentrated Treatments 32 Fl Oz - 441 Unique’s Marine Digest-It is specially designed to eliminate odor and backups caused by solid waste and residual sludge inside the entirety of your holding tank system lines and all. Marine Digest-It works with salt water and can be used in black and gray tanks. *Disclaimer: Ingredient disclosures are to the best of our knowledge based upon information provided by our raw material suppliers