I am using RO water for my newly started 100 liters aquarium and it results in a pH value of 6.0 to 6.2 which is too low considering that KH is 0.
In this case, I need something to raise pH (and KH eventually). The candidates for the time being are:
NaHCO3 (baking soda): Raises KH and pH but adds Na+ (sodium ions) to water which is not very useful.
KHCO3 (potassium bicarbonate): Raises KH and a tiny bit pH but needs to be used with care since adds K+ (potassium ions) in the water which is very useful in right amounts, but excessive use can be bad for fish and plants.
Do you have any other methods/formulas/combounds to raise pH in RO water in a controllable way without adding any unknown elements in my aquarium?