Is your feature request related to a problem? Please describe.
This backpack mod is often paired with survival mods which involve thirst (of which many involve drinking water from glass bottles). Backpack tanks are redundant in this scenario as it's required to place a water source to fill those bottles currently.

Describe the solution you'd like
Placing bottles in the tank slot will add or remove 250ml of fluid from the tank, if the fluid is water.