You will be better starting off with a tank of a minimum of 30 gallons. Nano saltwater tanks are unforgiving when it comes to little slips in maintenance or any problem. With saltwater tanks, you are best starting off with the biggest you can afford.
Before you start buying anymore equipment or setting up a tank; you should start with reading one or both of these books before you even start thinking about buying equipment or setting up a tank:
the new marine aquarium; Step-by-Step Setup and Stocking Guide, by Michael S. Paletta or
The conscientious marine aquarist: A Commonsense Handbook for Successful Saltwater Hobbyists, by Bob Fenner are about the best two books to buy on how to setup and maintain saltwater tanks. They will teach you a lot.