You can use fine grained sand (commonly referred to as sugar or oolite sand) on lower flow fish only systems where you only need about 20 times the display tank in GPH water flow. For reef tanks where you want 30 times your DT in flow for soft corals or 40 times+ for SPS coral, the finer grained sands get blown all over the place, so most of us use a courser sand (I like CaribSea Special Grade). To determine how much sand you need to purchase take the width and length of your tank in inches and multiply those two numbers together and then multiply that number by the inches of sand you want (2" is a good depth). Divide that number by 20 and you get the approximate dry weight of sand needed.
36x16x2=57 pounds of sand.