Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Coral sand is available in aquarium stores. It is used as a substrate in marine tanks. Entirely safe in fresh water tanks, too.
Limestone and related minerals (Dolomite, Calcite and other names) will also do this. Large chunks sitting in the tank are slow to alter the water parameters. Fine sand sized particles either as substrate or in the filter (in a bag) will have a much faster effect.
Oyster shell grit sold for caged birds is another material with pretty much the same properties.
Get one of those materials in a small amount.
Put it in a nylon stocking (I cut up knee his, and get 2 bags for a large tank, or 3 bags for a small tank out of one stocking).
Put this in the filter and monitor the GH, KH and pH.
All these materials are mostly calcium carbonates and magnesium carbonates. As they dissolve in water they add calcium and magnesium (GH) and carbonates (KH) to the water.
As the KH rises the pH generally rises, though when the KH is still so low other things in the water can control it. (organic matter, CO2)
You could add baking soda (as you are doing). I found it clouds the water only for a few minutes to an hour, then the water is clear.
Potassium bicarbonate is another option. You would use less of it, but it also will raise the KH.
When I want to do this, I make it a 2-part job:
1) Prepare water for water changes with the GH, KH and TDS that I want. Usually having the right KH means the pH will be in the right range.
2) If something in the tank is removing these, and the GH, KH or TDS is dropping, then add corrective materials through the week as needed.
GH- Adding GH booster can raise the GH. This does nothing about the KH. Seachem Equilibrium is one such product.