too many cooks.
High alkalinity will always reduce calcium levels in amounts that bring the supersaturation quotient back to levels that the conditions in the water column will support. This mean equal amounts of both calcium an alkalinity in terms of their solubility, adequate Mg++, adequate amounts of all the other conservative ion in seawater, full strength seawater, and shooting for calcium and alkalinity numbers that are appropriate to start with. ATO to maintain salinity between top offs will help a lot., especially if you are close to supersaturation due to inequality of one ion (i.e., if alk is 15 at S=32, and you increase your salinity to 35 by letting evaporation occur, then you've just increased the concentration by 9% of all conservative elements. This means that if you were close to supersaturation, then you may have exceeded it during the period of evaporation. This would represent an increase in alk of 15 to 16.5dKH in our example. At this higher alk, calcium would precipitate and drop both levels of alk and calcium until conditions were reestablished below the max solubility again.
Increasing alk is not necessarily equal to increasing pH. Adding a buffer to a barrel of RO/DI will change as the true driver of ph dissipates. It is the CO2 in the mains water that drives pH down, and in areas where the water is hard (high levels of dissolved CaCO3), the calcium carbonate tends to quickly exhaust the filters in RO/DI, esp. the DI section. At this point, the CO3 is converted to OH- as the pressure drops in the line and the CO3--->CO2. The extra CO2 degasses from the solution, leaving a relatively higher pH water behind. If allowed to circulate for 24 hours, even this will tend to dissipate, and the pH will tend to stabilize in that period.
Suggestions at this point:<ul>
<li>DO NOT ADD ANYTHING TO YOUR RO/DI, except salt mix for water changes</li>
<li>Collect your RO/DI 24 hours prior to your intended use and circulate in for 24 hours with exposure to outdoor air.</li>
<li>Brute trashcans on wheels: need I say more?</li>
<li>Set up an ATO device</li>
<li>Raise your Salinity to 35 PPT</li>
<li>Set your tank temp at 78<sup>o</sup>F. 2 degrees can make a big difference if you’re near supersaturation, and unlike sugar, calcium carbonate dissolves better at lower temps.</li>
<li>Perform a 50% water change now, repeat this again in a week. This will not only reestablish you conservative proportionality, but fix issues with imbalance of your calcium and bicarbonate/carbonate.</li>
<li>Consider adding kalkwasser to your ATO when you need a boost for your hermatypic supplements and get away from 2-part additives. Kalk = pickling lime and is ~$3 USD/lb at Wal-Mart. To teaspoons per gallon of RO/DI will go a long way and is a balanced additive.</li>
<li>Get a Pinpoint pH probe and some good (Salifert) calcium and Mg++ test kits</li>
<li>Buying a refractometer would be good if you don't already have one.</li>
<li>Get two in-line probes for TDS for your RO/DI. Place them at the output for the RO membrane (to watch for exhaustion of the membrane) ad at the final outlet for the DI columns (to watch for DI exhaustion).</li>
<li>Consider shopping for a CO2-driven calcium reactor, a pH controller, and a 20lb tank with a regulator. It is expensive to get initially, but cheap to run and doe the best job when adjusted correctly for large tanks with heavy hermatypic growth.</li>
</ul>
Adding buffer to water made acidic by dissolved CO2 will be a problem when the CO2 dissipates, as the acid will then be gone, but the buffering effect will still be there, tending to drive the pH to the high side and precipitate your calcium (both due to high pH and high buffer level). This makes addition of buffer not only wasteful and unnecessary, but derogatory to your calcium levels.
There is no need to add anything to your RO/DI water, if you live where the water is hard, it will help neutralize issues with organic acids. The buffer in your salt mix would correct for issues with pH, but you really do not have a problem if you circulate the water prior to use. I have more, but the thunderstorms here keep taking out my satellite service.
HTH
