I am sorry to be the bearer of bad news, but except for the few corals that do well under low light, you will be quite limited as to what you can have. A suggested
minimum would be 4 watts per gallon.
One of your tanks inhabitants can be rather "iffy" with corals. That would be the niger trigger.
I have a small niger in one of my coral grow out tanks and it is not picking on anything *yet*.
As for adding calcium, you did right by checking levels first. Salesmen are there to do just that, sell things..I amsure you know what I mean by that
I leave my lights on for 10 hours each day.