(11/22) Since the basil deficiency I have been waiting for this new addition. Like CaCO3 it is a natural product, from the rock know as dolomite, and completely none toxic. It is more soluble than CaCO3 and a 200 gram MgCO3 addition, added Friday (11/18), has proven to be more effective at adjusting and maintaining higher pH levels than a 350 gram addition of CaCO3. The 200 g MgCO3 addition raised the pH from 6.6 to 7.0; 350 g CaCO3 could only raise pH from 6.6 to 6.8.
(12/06) The 200 grams of MgCO3 (80 mg/L) addition added on 11/18 is continuing to provide a significant buffer and the pH is still maintaining around 6.8-7.0. Although the lower feeding rate is helping keep the pH stable it is still equivalent to 7 days of a 270 grams/day feeding rate.
Overall MgCO3 has proven to be an overall better addition than CaCO3 because of it greater solubility but both are still very important to the chemistry of the water. Too much Mg or Ca can cause cation based nutrients to become limited as ions with similar charges compete for plant uptake, for example to much Mg in particular can limit Ca uptake by crops. Not enough Mg can also cause ionic imbalance which contributes to decreases in alkalinity as Ca tends to combine permanently with carbonate; essentially the Mg keeps the Ca and carbonate from combining.
The main point to take away here is that too much CaCO3 or MgCO3 can cause water quality issues and it best to incorporate both into your water quality management strategy. When we were using just CaCO3 Mg likely became somewhat limited so by adding MgCO3 the ionic imbalance has been corrected and pH is being maintained over longer periods. This is also the most likely reason behind the deficiency in Basil.
Above is an image of the MgCO3 brand I am using in our research system. This particular brand is food grade for human consumption and can be organically sourced from dolomite.