Just to add to what Buck already said...
Depending on how hard your water is, you need to be careful what you are doing to the pH in a very small tank like that. I am sure it can easily be overdone...
A long time ago, when reactors were not known yet, we used a little piece of a branch of a lime tree (Tilia), peeled off the bark, dried it, and stuck it into airline. This creates a stream of very fine bubbles, which mostly dissolve on their way to the surface. Just a thought if you don't want to uglify your 5 gal with a big plastic reactor and a 200 gal/hour powerhead
Since there were no CO2 measuring kits, we also measured the pH of tap water, then blew with a straw into the solution, which added CO2 and changed the pH. Then measured the tank water, and it was supposed to be in between the two first measurements. But I don't remember the details or scientific background... well that was when I was 8 or 10 years old :mrgreen:
Right now my mixture is 1 (big) cup of sugar, a little bit yeast (was never able to exactly measure 1/8 of a tea spoon), and warm water. This lasts for about 2 weeks, and for my tank I am using 2 bottles, and one of them is refreshed every week. I don't use baking soda, because I have hard water and I think the sodium would be poisonous to the yeast cells.
Good luck...