Hold on a second or two. I didn't mean to imply that you should put baking soda in your tank water. The baking soda goes in the solution of your fermentation unit. It buffers the solution and allows the culture to work more slowly and last longer. How it works exactly, I can only speculate, but I think it has to do with the equilibrium reached in solution between carbonate anions and carbonic anions (dissolved CO-2) creating an environment that slows down the yeast metabolism.
Openning the unit will not tell you much at this point I suspect. Check the tubing that brings the gas to your diffuser for foam residue. Foam ups are most likely to happen a few days after start up if the unit is in a warm place or if the unit gets knocked over, with fluid getting into the diffuser tubing.
My dumbass cat, Wanker, has knocked over fermentation units while fishing in my tanks resulting in solution being blown into the tank with similar results to what you describe.
I recommend not adjusting pH or hardness of the water in your tank beyond the affect that increased water changes will have, as to do so may push your fish to the 'tipping point' that allows disease or other stress related factors to make maters worse.