First, and most important.
If you have the desire to watch things in your tank grow for years and years, you can't rely on luck. Some day your number will come up, and that will be it for your system. I know this because i rolled like this for 4 years.
Now i am a changed man.
What this means: on the back of many bottles there is a "beginner" area that pretty much will say something like "lol add one cap (approx.. wtf do you care, you are just throwing junk in your tank) pre 25 gal"
SO, never add chemicals that you don't test for.
i add:
seachem reef builder alkalinity buffer
aquavitro calcium
kent tech M magnesium supplement
seachem Prime (old habit, dies hard, i consider it jesus juice, whatever)
i test for:
magnesium
ca
nitrate/ite
amonia
phos
alk
I go to canada to pick up most of my chemicals. I am looking at the label of my tech M right now, 7.99 (ca) for a 16 oz bottle, and 12.99 for my 11.8 oz bottle of Calcium.
Depending on what you want to grow, how fast you want it to grow, and how healthy you want it to be in the long run.. you should be closely monitoring your alk, ca, and mag for sure.
in closing. This is not from some guy who has done everything right from the start and has to throw out 5lbs of sps frags a month because his system is rocking so hard. I got sick of pulling out 40 dollar skeletons and only growing mushrooms.
Oh, from day one with your reef, build a log book for testing, and a log book that you write in every time you stick your hand in the tank. Include details. Then set up a testing schedule (i am on every three days, mostly because i am borderline fish tank ocd and find testing to be fun) and stick to it. Your tests, if ran frequently enough will tell you that your tank is planning on taking a nose dive long before your lose that super cool blue acropora.
good luck, and again, if there is one thing i have learned, reef tanks are not for casual players. Dedication, and diligence makes it happen (or just lots and lots of throw away money). I wish i was you, asking these questions back in 2003