Think about it a steel tank mine 108 lp or 132 hp weights 41 lbs empty a 80 al 22 or there about. you lose 6 lbs on the belt but gained gear weight. I do not know anyone that dives steel that lost gear weight by going to steel.
I dive short or shallow dives with my al 80 and can get 60 to 90 minutes. If I am diving 50 to 100 ft I like my 100’s or hunting lobstering, diving wrecks I well drop with 132 ft of gas. A pony tank is a good idia if you are doing 100 or wrecks at any depth. Start with a al 80 and add on. all my friends that dive steel also have at least 1 al 80. Have fun.