Step 3 – Determine the alkalinity/calcium demand of your tank. After determining how much water evaporates from the tank and you set up the auto top off system, the next thing to do is determine how much alkalinity/calcium the tank demands. To do this, turn on your auto top off system so that the water level in the tank stays constant. Measure the alkalinity and calcium in the tank and adjust these parameters to desired levels. To adjust the parameters, visit http://reef.diesyst.com/chemcalc/chemcalc.html and use one of the products in the drop down boxes to adjust whichever parameter needs adjusting. I recommend using baking soda (inexpensive) to adjust alkalinity and Seachem Reef Complete (it has the highest concentrations of calcium per volume) to adjust calcium. For this first set of adjustments, I also recommend adjusting these parameters independently instead of using a product like limewater that adjusts them both at the same time. Re-test the parameters after you think you have them where you want them to get a solid reading. Now wait 24 hours and measure them again. Record these values and adjust them again to the desired levels. Wait 24 hours and measure and record them again. Do this at least 3 days in a row to get a good set of average values. So if you have your alkalinity adjusted to 10.0 dKH and it drops to 9 the next day, you know your tank is demanding 1 dKH per day.