A plasma tank was a storage vessel for plasma. Damage to the tank or a plasma conduit connected to it could be a serious condition if the fire resulting from such damage could not be put out with a fire extinguisher.
In 2370, Kira Nerys and Jadzia Dax were forced to jettison the plasma tank of their Bajoran raider shortly before it crash-landed. This was due to weapons fire from other Bajoran vessels controlled by members of the Alliance for Global Unity. (DS9: "The Siege")