Hi All I found this on the net: Full article on http://www.aquahobby.com/articles/e_co2.php I have just copied and pasted the section I would like to query on: Has any body ever tried this and is it effective? How to make the injector OK, if you've read and understood what'd been discussed above, then you're ready now to learn how to make and correctly use a DIY CO2 injector. which as we've mentioned is extremely simple. Material for the bottle: 1 two-liter plastic bottle (with plastic bottle cap); 1 thin plastic tube (the kind used in tanks with air pumps); 1 airstone (optional); Aquarium silicon sealant. Procedure: Make a hole in the bottle cap, such that the tube squeezes tightly through it. Insert the tube until its about 3 cm below the cap, and then spread silicon around the hole to seal it. Let it dry for a day. The plastic tube should be long enough to go from the bottle position down to the gravel in the tank. Put an airstone at the end of it if you want thinner bubbles. Material for the mixture: 1 teaspoon of yeast; 2 or 3 cups of sugar; Â½ teaspoon of Baking Soda; 1.5 liters of water (preferably filtered and free of chlorine). Procedure: Completely dissolve the yeast, sugar and baking soda into the water. Pour it into the bottle and seal it well. The production of bubbles will generally begin in an hour or so. A well made injector will keep producing bubbles for about 10-15 days. We have presented here only a basic description of how to set one up. There are many variants that can be found in articles around the net. There are also many comments and details that can help the user to improve or optimize the DIY injector, but this is also subject for other articles. The main reason for this one was to clarify and prepare anyone interested, so that he or she may make the decision based on objective information, not on fashion!