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. They 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 an E-Tank. 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 since they are already full, they do not collect energy from energy Capsules.
One of the Subtanks is in the Bio Lab, after you finish the Waterfall Ruins go to the Bio Lab and attack all the little electricity containers to the small capsules. Up a ladder there will be a door which will be blocked by a mechanical "door" which will only open once you put all the containers in the capsules. Once you're in the room A-Trans to Argoyle and summon Ugoyle you should see a Subtank enclosed, simply make Ugoyle touch it and you got a Subtank.
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