You can get a 2L bottle, drill a hole in the top, and attach some 1/4" vinyl tubing or something of the sort using silicone. Add like six cups of warm water, 1 cup of sugar, and a teaspoon or two of yeast. Just dump out about half every week or two and replace with sugar water. The yeast cells multiplying produce plenty of co2 which comes out through the tube, really cheap way to increase co2 levels. I used to do this for fish aquariums with lots of live plants. You'd probably want to put the end of the tubing near the top of the plants if you do this, since I'm pretty sure co2 is heavier than normal air.