Don't mean to be harsh but you're setup is not optimal for a reef. I am not saying you won't be successful, just that you have some hurdles.
First is the tall tank, very little water is actually touching air, this makes gas exchange difficult. I would suggest a power head to help things out. As you are in a 20 a small one should do the job nicely.
Second the CC will trap detrius and uneaten food, making nitrate control a problem, adding fish will make this worse. A DeepSandBed of at least 4 inches will help with nitrate control.
I know some will argue (hi andymi
) but a protien skimmer would also be a good addition.
But not to get the cart before the horse...
What do you have in your tank exactly now?
Live rock? What is your lighting? How long has your tank been running? What are your tank readings?
Nitrates a problem now?
Not trying to rain on your parade, just trying to find out how things stand.