First of all welcome to the board. You will get tons of good info on this board, so you are in the right place. To help with some of your questions, you do not need your lights on that long. Twelve hours or less is fine. That algea growth will continue with that much light cycle. Second, as far as the additives to the water go, you will not need that stuff for a while. You may never need it. Most of the elements you need in the water will come through water changes. The salt mix has everything you need. Their are some exceptions to this rule, such as, when you get into SPS corals and clams that need cal. and alk. Don't even worry about that now, these are advanced topics that come with time. Some comments...
Are you using tap water? If so, you will want to switch over to RO/DI. You can get it from grocerie stores for like $.25 a gallon. This will save you trouble later and you won't need to condition the water. In my opinion the less artificial stuff you put in your tank the better.
You need to look into stronger lighting if you want coral. Their are lots of types of lightings and it can get real confusing, but for a twenty gallon you will most likely want PC or VHO lighting. I have the corallife 130w 24" PC and I really like it. It is inexpensive too ($110+). If you shop around though you can find tons of lighting and you will just need to decide what you need for your setup.
I hope this helps. I know it is kind of long, but I am a talker
AJ