+1 on the nitrite, which is nearly as toxic as ammonia. GET RID OF THE NITRITE (and boost your dKh!) and your corals will likely stop dying.

To make it easy, I would use some commercial solutions to raise the Alk and Mag levels... they seem to be pretty straightforward with their instructions on the bottle, though I've only looked at very few kinds of dosing additives. Follow instructions and you should be fine.

Question, how old is your tank?

I've used drug store epson salts in the past for boosting mag. For alkalinity I like baking soda. Heres a calculator that might help you:

http://reef.diesyst.com/chemcalc/chemcalc.html