That sounds like a big bag, a 1' x 5' rectangle that 1' deep?! Are you sure it's not .5 cubic feet? That would be a 1' x 1' square thats 6" deep...Probably 50lbs bags. Little bit of math can tell you how many inches of depth you will have. Measure your tank length and width in inches then divide by 12, multipy that together gives you SQUARE feet, the height is what makes it CUBIC so a 12"x 12" square is 1 Sq Ft. Then with a 3" depth is .25cf, 4" is .33cf, 6"is .50cf and 12" deep is 1cf... Get it?
Assumeing your 75g is 48"x 18" thats 4' x 1.5' = 6 sq ft, 3" deep should be 2 Cubic foot. if your bags are .5 cf then you are there with 4 bags.... At least that sounds about right to me :lol: