look at a pony tank instead of spare air (we call it death in a can) it really dose not give you enough air beyond 50 feet, when your worked up and preathing heavy, plus they are spendy for what you really get. I have a 30cu pony it matched the hight of my hp80s well but a 13 cu and a very basic 1st and 2nd stage are not that spendy. Just a idea. My dive buddy used to carry spare air till we did some dive drills with it and he was suprisec how little air was available,