The nitrogen cycle starts with ammonia excreted both by fish and the decay of once living material. Bacteria eat ammonia and give off nitrites. A different bacteria then eats the nitrites and gives off nitrate. Nitrate is also called "fertilizer" and all manner of plants LOVE it. Dissolved CO2 in water is also called carbonic acid (think soda). An increase in dissolved CO2, either from surface agitation or from respiration by fish can acidify the water. If there are fish in the water, there is likely to be an over saturation of CO2 in the water as it is, and the aeration will help to lessen the CO2. That is obviously not always the case though. Plants will suck in a ton of CO2, which can raise the PH of you water by removing the carbon from the carbonic acid (which just gives you more water, not straight up acid). Air stones provide a double whammy as far as the gas exchange is concerned. They first circulate the water, which reduces the time it takes air soluble chemicals to disperse from the lower reaches of the water to the higher. Otherwise, in theory at least, the Ph at the bottom of your tank could potentially be higher than the top of the tank. This is also why fish have an easier time breathing in still water. Second, the gas transfer happens wherever the water meets air. The total surface area of air and water is the concern, which means waves have additional gas transfer ability over still water.
My major concern with adding a lump of baking soda is a quick PH change can, and will, kill just about everything in tank, including the nitrifying bacteria. Slow and steady is almost always better. Also, baking soda is sodium bicarbonate, which in the presence of say, carbonic acid and clorine will produce salts. Not an incredible issue, but if you do this enough your tank will become saltier and saltier without water changes. And considering a water change would probably fix the problem anyway, it seems frivolous.
In conclusion, god I need a life. But in reference to the original post, you didn't screw up, but there are better, and safer ways to solve the same issue.