Im thinking of buying some sand for my tank but what size grit should i buy 16 or 20 ?

I use PFS pool filter sand in all my front tanks. Comes in brown or white in a 50 pound bag for about $10. Works pretty good. Some people have bought sand at a building products store like Home Depot. It to was priced pretty good.

The pool filter sand is a heavier grain so it doesnt hang in the tank like play sand. Play sand will hang in the water after its stirred up and get into your filters, ruining them. I use the pool filter sand am I am very happy with it.