Welcome to the board, and contrary to a previous comment, 20G is PLENTY for a reef tank. I run a 20, and probably some of the nicest tanks i have seen on this board are 20 gallons. But, you will want larger, but IMO a 20 is a great place to start. Small enough for you to establish excellent husbandry, but not too small. Best of luck, and Please Please Please Ask Before You Buy!!