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")