Most charter operators do not let me dive twice on the same tank even if I surface with more than 2/3 of air. However they're OK with my 2nd tank, which is smaller & has less air than what I came up with in the first one.

This would be the 2nd dive on the same wreck and with the same dive plan.

What's so annoying about this is that I still pay the same for the tank to be topped off at the end of the day.