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Sub Tanks are special containers that are found in some levels in the game. There are four in total, and require sharp eyes to find them. Their speciality is to store extra life energy that you pick up when you have full life energy. its capacity is the equivalent of your character's maximum health with all of the Life-ups Collected, which is 32Hp bars. To use a Subtank's stored energy, you select it from the Menu Screen under the heading "Subtanks".

There is also a similar Item that you can find after competing a few sidequests called the Lifetank. It functions in a similar way to a Subtank in that you can recover all of your health, but you can only use it once before you have to buy another one, and they do not collect enery from energy Capsules.