I wish you much success as you begin your 20g tank. Because it is small in terms of saltwater aquaria, strangely your workload will be considerably greater. Smaller SW
tanks require a lot of monitoring and maintenance. If you are successful in maintaining your hex tank, you will have a lot of good experience under your belt; if you have problems while trying, the same will hold true.
You might check on the net for NANO SW
tank sites for further encouragement.
Good luck, and please keep us posted. You will find lots of good info in the archives, too.