The High Capacity O₂ Tank is an upgrade to the Standard O₂ Tank that is crafted at the Fabricator, being the only upgrade not requiring a Modification Station. It can only be crafted after the player has acquired a Standard O₂ Tank. While equipped, it boosts the player's maximum Oxygen supply by 90 units for a total of 135 units, but slows their non-Seaglide 5.74 m/s Swimming Speed by -0.4 m/s underwater and -0.53 m/s on the surface.
The player must equip the High Capacity O2 Tank in the tank slot of the Paperdoll UI in order to benefit from it. While not equipped, it takes six slots (2×3) in the inventory.
It can be upgraded to the Lightweight High Capacity Tank or the Ultra High Capacity Tank at the Modification Station. Neither can be upgraded into the other. The Lightweight tank carries the same oxygen as the High Capacity tank, but has only 1/4 the movement penalty. The Ultra High tank has twice the oxygen of the High Capacity, but a 50% greater swimming penalty, and is the only tank with speed penalty when in inventory as well as when equipped in a Paperdoll slot.
When an O2 tank of any kind is fabricated or upgraded, it defaults to being empty and it must be equipped in an atmosphere to fill it. This is important if the player intends to carry multiple tanks for deep diving without a vehicle.