- For a 9″x48″ tank, you’ll need the - 1 cu ft (cubic foot) bag - 32,000-grain.- A 10″x54″ tank requires half a measurement up from the 1 cu ft - 1.5 cubic feet - 48,000-grain.- Tanks of 12″x48″ in size want the 2 cubic ft - 64,000-grain.