A spray tank was a component that could be installed on board Constitution-class starships.
Montgomery Scott installed a number of spray tanks on board the USS Enterprise in 2268 to assist in the ship's mission to decontaminate Ariannus, which was threatened by a bacterial invasion.
The spray tank applicator, which was deployed from a dorsal section of the ship, applied the decontaminate to the planet's atmosphere from orbit. The procedure was repeated for a second orbital run for maximum effect. (TOS: "Let That Be Your Last Battlefield")
The tank's spray applicator was a new special effect created in the 2008 remastered version of the episode.