god i would kill to get a 150w hqi over my 14g, i just can afford it
to answer your question in a way your not looking for boxcar182, you have alot of reading to do! first you need to know what the corals your trying to keep are going to need, then you can decide what to add as far as suppliments are concerned. as a beginner, sps in a 20g tank shouldnt have been your firts choice, but what the heck, your already on your way here.
youve got plently of light, just make sure to change out the bulbs every what is it, 12-14 months?
youve got a skimmer, learn how to get it dialed in. ive heard that particular skimmer is hard to get working efficiantly.
b-ionic is a 2 part calcium/alkalinity suppliment. you use equal parts of each suppliment and its supposed to maintain a balanced ca/alk. you jus have to keep testing to see how much your putting in, and how much your losing day to day. unless you have tons of sps, you sohuldnt have to use very much to maintain your levels. always start off using less that the bottle says to add...
dt's makes the same thing but it has a magnesium suppliment that goes with it, so its a 3 part system.
i would say dont bother with anything else. do weekly water changes of around 2-4 gallons, that should do alot to help maintain trace elements, and keep up with water quality.
calcium in the range of 380ppm to 440ppm. :edit: -when you find out where your calcium is, there is a balanced alkalinity to match. find it on the calculator in the link.-
magnesium 1230 -1400, ive got a magnesium quark, dont get me started!
if you have fish, dont over feed them. if you see detritus building up on the sandbed, siphon it off. good husbandry is key.
this link should help you figure out how much you need to dose to get from a certain level, to another level...
http://jdieck1.home.comcast.net/~jdieck1/chemcalc.html
go get some books......