[coralreefer]

CaseyH,
You do not mention what the pH of your water is. If you have a low pH, the H+ ions will react with any carbonates in your tank. This will liberate Calcium in soluble form. I test freshwater calcium using versenate powder and reagent grade EDTA (the same EDTA is used as a food preservative).If the source of the Ca is your freshwater you can test for this by using a sample (100ml) of your replacement salt water before adding it to your tank. Mix baking soda into the sample in small amounts (use a paperclip that is flattened on one end). After each baking soda addition, agitate vigorously and allow sample to set undisturbed for a few minutes then retest the sample. If this technique lowers the pH too much, use sodium carbonate. The chemistry is almost identical, the sodium carbonate will not reduce the pH but the reaction will take longer in your test sample. IF YOU USE TOO MUCH OF EITHER OF THESE SODIUM COMPOUNDS YOU WILL DEPLETE THE REMAINING CALCIUM! A little goes a long way. You may find it easier to dispense the baking soda or sodium carbonate by dissolving them fist in fresh water before adding to your test sample.
If your makeup water is the source, try using a membrane in your RO with a smaller pore size. Hope this helps without overwhelming you with a chemistry lesson.