Really basic chemistry but CaCL2 calcified with Na2CO3 will leave CaCO3 and 2 NaCl that will raise salinity. I use about 1 gallon every 6 weeks now so some water change to account for this will always take place in my tank.Traditional 2-part dosing also should consider a bit of water change from potential to build up salinity.
How is this.
I will increase feeding a bit more.
I think your doing a bit of hand waving to say dilution is not a factor of nutrient export still in a natural reef. If the gradient is between ocean and reef is 5 fold for nitrogen then it will take place and difficult to quantify. I am also certain some DOC is less viable than other DOC as food, at least for what is present in most tanks.
What has become very dusty and appears more at some times of the day. Growth rate of coraline algae is less but over all calcium demand is the same so far.