Buffered solutions are the best way to maintain pH. To avoid CO2, dont use any carbonate or bicarbonate buffers, Seawater has a pH of about 8, so phosphate buffers are most common for this pH range. You can keep the concentrations of the phosphate salts low if you do not need much in the way of buffering capacity.

in order to give good advice, I would need to know the setup of your water treatment. What volume are we talking about, which organisms are in the aquarium ?