If your freshwater tank has a low KH, you can use baking soda (sodium bicarbonate NaHCO3) to increase the carbonate hardness (KH) of your water. This is also called buffering capacity or alkalinity. Adding baking soda will not increase your general hardness (GH). KH should be at or above 4.5 DH / 80ppm to keep pH from becoming acidic. I buffer my tanks up to 7dh to keep KH above 4dh by the next weekly partial water change.
When you dose the tank, dissolve the baking soda in water you've removed from the tank; then add the mixture slowly to the tank stirring to disperse. Shoot for raising pH by no more than .4 per day.
Google search: KH pH and "baking soda"
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