The TST Marine Head Drop-Ins help control unwanted odors in your holding tank. The easy to use Drop-Ins contain a special chemical that helps break down waste, and a blend of citrus-scented surfactant oils that trap unwanted odors in your tank, making your boating experience more enjoyable. The Drop-Ins are marine approved, non-formaldehyde and safe for all septic tanks. Each reusable, recyclable box comes with 30 Drop-Ins.