How Much Sand Is Required For This Filter?

Sometimes you can buy sand, gravel, and rocks in "cubic feet" or "cubic yards" (called just "yards" in the industry). A cubic foot is one foot high, one foot wide, and one foot long, and equals 1728 cubic inches. That would fill your tank bottom to a depth of about five inches, which is probably more than you want. A cubic yard is 27 cubic feet (3feet x 3 feet x 3 feet), or 46,656 cubic inches, which would probably be a lifetime supply for the average aquarist.

How many pounds there are to a cubic foot or yard depends on the material. A cubic foot of pumice weighs a lot less than a cubic foot of granite.
This source says a cubic foot of pool filter sand wieghs 82 pounds:
http://www.ultramodern.com/Bulletins/BUL…
So 50 pounds would be about 50/82 of a cubic foot, or .61 cu. ft.
That should fill the 29-gallon tank to a depth of about 3 inches, if there are no rocks, driftwood, caves, plant roots, or ornaments taking up space on the bottom. If there are any objects on the bottom, they will fill some space, so 50 pounds will end up making a deeper layer than 3 inches.