Many people start off with a 15L or 12L cylinder and then, as they venture deeper/longer, they realise the need for either more gas or a back-up gas supply. Most therefore opt for a “pony”. A “pony” is a small (usually 3L) cylinder that one fits a second regulator and contents gauge to. I have nothing against “ponies” per se but I do have some comments on how they are set-up and where they are put.