We've all seen that pesky trail of tiny bubbles rising from the tank valve. It's lost air, probably escaping through a bad O-ring. Inspect the tank's O-ring before attaching your regulator first stage. It should not be dry, flaky or cracked. If it is, ask the dive operator to change it. Also check that the O-ring seat is not dented or misshapen. That could cause the same leak.
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