Johnny the O2 tanks don't stack if I'm reading your post correctly. Its just one high capacity or whatever kind of O2 tank that you can use at a time. The one you're carrying around in your inventory to switch out with the one your wearing, it doesn't work that way. Unfortunately its not an extra resource so to speak that you can use. Its just a redundant item taking up space in your inventory. You can only use one O2 tank of whatever version at a time and you'll only have as much air as the version type gives you until you go to the surface to get them refilled. When you get a Seamoth, jumping in the cockpit recharges your personal O2 tanks too. Its a little, mobile atmosphere and safe spot for when you really get to exploring deeper areas. But unfortunately you can't just carry extra O2 tanks with you like you are thinking, which is a good, sensible idea but the game doesn't let that tactic work in this world.

That being said, I never tried it so maybe they do let you do that and there's a way to do it? I don't think its the case though knowing how intricately designed the game is around the balancing of O2 needs with exploration/underwater risk taking and challenge.