This article explains how to make an easy DIY CO2 injector for your planted aquarium. All you will need is:
(1)empty plastic (2 liter) pop bottle
(1/4-1/2) teaspoon of yeast
(1 1/2) (about) cups of sugar
one-way check valve
aquarium-safe silicone sealant
Now, to put it together. Nail or drill a hole into the bottle cap large enough to fit the airline tubing into, and feed about half an inch of the tubing into the cap. seal the hole with tubing in it on both sides with the aquarium-safe sealant. Leave this until it's dry (about one day.)After it's dry, pour about 1 cup of warm water into the bottle and add the yeast. Cover the top of the bottle with your hand and shake well until the yeast is completely dissolved. The warm water dissolves the yeast faster. Now add water that's about room temperature to the bottle until it's about 3/4 full. Add the sugar and shake again until the sugar is completely dissolved. Screw on the cap with the airline tubing and the bubbles will start being produced faster after a day or two.
You can dissolve bubbles into the tank water two ways. One way is to feed the tubing into your filter intake (without an airstone,) where the bubbles will be shredded into smaller parts by the impeller, be eposed to a strong current, and may get caught in the sponge where they will dissolve, or you can build a reactor, which does take up some room in the tank, but once your plants start growing it can easily be hidden. All you need for a reactor is
For my 20gal, i used a small Naya water bottle, and put it were it would be hidden behind a rock with java fern growing on it, but would still get adequate water circulation. What you do is cut out panels of the center of the bottle but do not cut past the top 1/4 of the bottle, and leave a rim on the bottom. Tightly screw on the top lid, and place it in the tank, weighed down by rocks. Put the airline tubing and airstone coming from the CO2 bottle into the reactor and your set.
The duration if this fermenting mixture will produce CO2 for about a month. The length of CO2 production is reliant upon the amount of yeast and sugar in the mixture. More yeast will produce more CO2 but will consume the sugar faster, so it won't ferment for as long. Less yeast will take longer to exhaust all the sugar but it wont produce as many bubbles per minute. It's up to you to experiment with.
**note: the addition of CO2 to the aquarium will cause pH levels to fluctuate during the day, when plants use CO2 for photosynthesis, and during the night, when the plants aren't producing O2 and aren't using CO2 either.