I started doing this about a year and a half ago, so obviously I cannot say it got rid of my candida overgrowth as I am still here and fighting, lol. But, it did have some benefits as well as problems.
Yes, it is plain baking soda.
First of all, you can get this affect with alka seltzer also, which can be convenient. 2 tablets in 8 oz water. However, there are a few different types of alka seltzer out there, and alka seltzer has its down side also. First of all, forget all the ones with added ingredients for colds and stuff. Too much gunk. Original alka seltzer works great, but do beware that it also contains aspirin, which can irritate leaky gut and end up making things worse, especially if you do this daily. The formula of alka seltzer is aspirin, bicarb of soda, and citric acid . There is also alka seltzer gold which does not have the aspirin, but does contain mallitol (a Sugar alcohol), which I personally found counter productive to candida healing. However, I did develop some stomach lining irritation (found during an endoscopy), which can lead to ulcers, and while I have no proof it was from the alka seltzer, my doc seems to think its very likely the cause due to the aspirin.
So, then I looked at the formula and saw the ingredients, sans aspirin, were very easy to get ahold of. Bicarb is baking soda, but I made sure to get a natural brand (I use Bob's Red Mill brand) that has no alluminum. The other ingredient is citric acid, and you can get a tub of that from Lucky Vitamin online. They sell the NOW brand of citric acid and it is pretty cheap for a huge tub that will last a year or more. I have not seen this in any stores. So, you can make your own with the baking soda and citric acid and it is very cost effective, whereas alka seltzer is kinda pricey for a box of 18 servings. The reason for the citric acid is that the baking soda (a base) uses the citric acid (an acid - duh) as a catalyst. In other words, put the base and the acid together and you create a reaction - you need that reaction of base and acid for it to work. However, just plain baking soda is probably fine if you are already fairly acidic (look at "Ask. Moreless" for better explanations of acid/base interactions) and you probably can do ok with just the bicarb (baking soda), though I have not personally tested just the bicarb alone.
I have found that 1 tsp. baking soda and 1/2 tsp. citric acid is the ideal ratio. I figured this out based on the size of the alka seltzer tabs I was trying to mimic, and also the taste/effervessence of the potion I made compared to what I would get from alka seltzer, and this matches perfectly.
One thing to keep in mind is a problem I ran into, but it can be overcome. After doing this for a while I developed a gut candida flare. I suspect it was because the stomach is SUPPOSED to be acidic (unlike most of the rest of the body), in fact an acidic stomach will kill candida in the gut, and if there is not enough stomach acid candida will gain a hold there. The bicarb reduces stomach acid. So, what I did was begin adding in HCL (hydrochloric acid) with my meals, to increase stomach acid for digestion, and I also cut back on how much of the baking soda/citric acid I was doing from about 4 x/day to 0-2 times a day. Doing this also reduced my candida population quite a bit. I clearly needed the stomach acid. So, timing and quantity of the bicarb mix is important and probably varies from person to person.
So, if it did not kill candida, what did it do for me? When I was feeling overly toxic from detoxing, with brain fog, lack of energy, etc., it really helps to clear out the acids that are causing that, and I would feel physically and mentally much better, and be able to work, etc. I especially found it helpful, and still do, if I wake up in the morning feeling particulalry awful. Usually just a couple of sips will do the trick for me, but standard advice I have read says sip slowly over a 30 minute period, as it only works against the acids as long as it is in the stomach, so gulping down like one might do if one were trying to cure a headache will not help as much.
What I would do is buy a small box of Original alka seltzer and try it, on an empty stomach. See if it makes you feel better. Then if it does you can go and order the citric acid and get some natural baking soda - no sense going out of your way if it does not help you.
Oh, I still keep some real alka seltzer around. The packets are convenient to take with you when you go out and can be dropped into a bottle of water easily. I only use alka maybe once or twice a month now, in emergencies.