Thetford Tank Blaster Holding Tank Cleaner works in both grey and black water holding tanks. It is safe for RV toilets, holding tanks, and septic systems. This two-in-one tank cleaner features a unique, dual-action formula. The cleaners lift deposits from the tank walls. The super-concentrated enzyme blend actively attacks and breaks down loosened deposits and prevents clumping, clogging, and tank residue. Micro-foam action improves surface contact with vertical tank walls. Special additives tackle hard water deposits on sensors.