The bulk update request #9008 is pending approval.

create alias air_tank (449) -> gas_cylinder (11)
create alias gas_tank (221) -> gas_cylinder (11)
create implication helium_tank (231) -> gas_cylinder (11)

Reason: All of these tags refer to the same thing: a cylindrical tank storing pressurized gas. They also share common themes. The reason for choosing gas_cylinder over gas_tank is because that is the proper term for a cylindrical tank storing pressurized gas as well as to disambiguate the tank tag