Pentane is gennerally not corrosive to the steels used in API 650 type aboveground storage tanks. However, I'd add 1/16" minimum becasue the tank inside will be exposed to air as you fill and empty, and most air contains some moisture content which can condense on the tank and fixed roof. If you are in a tropical marine environment exposed to moist, salty air then maybe 1/8". If you think there will be water in the Pentane (which may be ahrd to avoid depending on your feed stream), follow LittleInch's advice and coat the bottom and 36" up the tank shell.
If you are getting the Penatne directly off a refinery process it may be warm enough and have a high enough TVP to require a chiller to be able to go into a floating roof tank.