NaHCO3 v.CaCO3
Hi Tug, Philosophos,
While not a brackish water expert, I have kept brackish water tanks for 20 or so years, though planted, specific has only been a few years.
Normally I do not concern myself too much with pH drops associated with CO2 injection.
In your case, I will make an exception, lucky you!
I think 4 dKH is a really low amount of buffering for a brackish water setup, I would like to see more like 12-20 dKH. Baking soda, NaHCO3 is the usual choice. It will raise pH through addition of Hydrogen.
I prefer for this purpose Calcium carbonate, CaCO3 this increases carbonate and calcium so GH and KH will increase.
By increasing the KH and GH with the addition of lime instead of baking soda has the advantage increasing the buffering capacity and not simply raising the pH. This may seem a distinction without a difference but the pH drop you are experiencing is the addition of a weak acid to a water column that has insufficient buffering.
As Philosophos points out it is the osmoregulation of your with which you are concerned.
An article that is a good (I think) explanation of alkalinity versus pH.
MARINELAND.COM - DR. TIM'S LIBRARY
I hope this helps or better yet someone who has a vague idea of what they are talking about shows up.
Biollante