I've found that these plastic adapters aren't well fitted, and often leak out.

I bought this adapter, then removed the valve head of a Sodastream tank with a crescent wrench (as per this video). Screwing the official valve into the adapter creates a perfect adapter that works with a standard hose and tank.

(Just be sure to install provided washer and O-ring between the adapter and the valve; otherwise it leaks like a sieve.)