What he said.

my lighting is 150w halide over a 20 gallon. the only thing i have never checked is my magnesium. i do have some coraline on the rocks that it came with but it has not grown at all. i don't know what to do

I started adding about half the recomanded dose of Purple Up to my tank once a week and i have been getting pretty good growth from it.

i do a half dose twice a week . i also drop my calcium dosage. its hard to say how much aas that would depend on your tank size and what your using. just be aware that purple up does have calcium in it.

I use Purple Up in both of my tanks
One tank has 175W MH lighting and no coraline the other has PC lighting and getting lots of coraline growth.