its hard to guess how much i need to buy, considering most 55 gallon tanks are longer and shallower , im sure the 2 pounds per gallon rule doesnt fly with this shape of tank..... its 70 square inches of bottom space
Just do the math. This assumes that it is an equilateral hexagon, that is all sides are the same length. Lets assume each glass panel is 12 inches. (I really have no idea what it is on a 55 gallon.) The hexagon makes six triangles, each one meeting in the middle. I'll save you some boring math, but the result is that the area of a equilateral hexagon is 2.59 times the square of the length of each side. So if your panel is 12 inches, the area is 372.96 or 373.
We can now go to cichlidforum.com, and check out their sand depth calculator, which is found here.
Although your tank is a hexagon, we know the area, so all we need to put in is a tank with the same area as yours. The easiest solution is one tank that is 373 inches long, and one inch wide. We can then put the sand depth as 1 inch, and it gives us the answer 21. So we need a 21 lbs of substrate to give us one inch depth. Now this assumes the length is 12 inches, which I just made up, (and it assumes my math is correct), but with the math in front of you, the calculations should be easy with any length for the front panel of glass, assuming all sides are the same.