Commando Black Holding Tank Cleaner from Walex Products Company has a super-concentrated enzyme formula that fights against tough buildup and unpleasant odors in the black holding tanks of restroom trailers. It works by penetrating deep into tank walls to remove waste and paper buildup, cleaning in as little as 12 hours. It is available in bulk tubs of 40 easy-to-use, dissolvable packets. The packets are dropped into the tanks, and the tanks are then filled with water. The recommended usage is two to four times per year, but it can be used more or less frequently if needed. 800/338-3155; www.walex.com.