First of all, my recommendation is not dose anything you can't test for.
In my opinion, the first thing to do is to establish your consumption rate. So do a full set of tests on day one, recording the parameters. Then let it run for a few days and then do another set of tests. You'll then get an idea of how much of each element was consumed by the tank over the number of days between tests.
So if your first test was 400 ppm and the second test was 350 ppm after 5 days, you consumed 5 ppm per day.
There are several calculators you can use to determine how much to dose to get and maintain your target numbers.
That's my two pennies anyway!
