All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
According to the general rule in Sharia, Zakah is obligatory on everything intended for trade. Allah the Almighty says {what means}: " O ye who believe! Give of the good things which ye have (honourably) earned" {Al-Baqarah, 267}. Mujahid commented, "This refers to trade." {Tafsir Ibn Kathir, Vol.1, Page 697}.
Regarding the ruling of Zakat on gas cylinders, the following details apply:
The first scenario: If the gas warehouse is only operated in filling gas cylinders and then selling them filled, replacing them with empty cylinders, and thus the trade is confined to selling the gas substance filled in the cylinders only, then Zakat is obligatory on the value of the gas substance. There is no Zakat on the value of the empty cylinders because they are considered containers for the merchandise, and the condition of growth (nemā') is not fulfilled for them. This is based on the statement of the Prophet, peace be upon him: "There is no Sadaqah (i.e., Zakat) on a Muslim's horse and slave." {Narrated by Al-Bukhari}.
Imam Amr ibn Shu'aib Ash-Shafi'i, may Allah have mercy on him, said: "Zakat is only obligatory on growing wealth." {Hashiyat Qalyubi and Amira, Vol. 2, Page 29}.
Accordingly, at the end of the year, the merchant assesses the price of the gas present in the cylinders he owns, and adds to it whatever other cash amounts he possesses. If the Nissab (Minimum amount liable for Zakah) is reached, it becomes obligatory for him to pay Zakat on the total amount at a rate of 2.5%.
The second scenario: If the gas warehouse sells both empty and filled gas cylinders, and earns from both, this makes them items of trade. Therefore, Zakat becomes obligatory on the value of the empty cylinders in addition to the value of the gas filled in them. This is because, in this case, the cylinders are being used for trade. And Allah the Almighty knows best.