Did one of these for the pocket watches, and was originally undecided on whether or not to do one for tanks too. But due to popular demand and some helpful contributions, here it is at last: the helium tank guide!

The tanks are used to increase your maximum carrying capacity, which can let Cynte carry more ammunition and healing items on him at any given time. Recently, an update added an achievement for getting all five helium tanks so fortunately this guide's already got you covered! Better yet, the achievement is only for five helium tanks total so that means I already covered all five already and don't need to do another scavenger hunt.

Impossible to miss. Found along the path through the level with the note explaining how to craft them, right after flipping the switch connected to the red wire.

Right before reaching the terminal that removes the elevator lock, there is a split in the path and you can go either to the left or the right.

Take the left path and flip the switch to close the door you just went through to open up a secret tunnel to the next helium tank.

The next helium tank is acquired after delivering the second bio sample to Karis. It'll be among the supplies he gives Cynte.

Like the other bio samples, this one shows up as a pink dot on the map. It's in plain sight around the last save terminal in the area.

Massive shout-outs to NagarJuna Cement for cluing me in on the floor for this Helium Tank, as I otherwise would've never gotten it.

This tank is found in the lower-right section of the map in one of the side-tunnels you can go into while you are being chased by the first Hydra but just before you can switch off the lights. It won't up up until after you turn off the lights though.

Go into the bathroom in the bottom-right side of the area. There is a lever to flip under the sink that opens up a tunnel going north. The Helium tank is there.

With the last helium tank upgrade, you should get the pride and satisfaction(tm) of gathering them all up, plus capping out your inventory space to a full 1600 units

... Or is it?

Actually, as of the most recent updates adding a Helium Tank achievement, I think now we're at the point that we can safely say that yeah, this is it. Achievement is for five tanks, we've found five tanks. Mission accomplished, everyone!

Big shout out again to NagarJuna Cement for convincing me to go back and track down the tank on floor 25.