Did not get good CO2 production out of these (I got 2 when I bought used tanks).
I get much better production with a 2 liter bottle, DIY set up. I used the same recipe in the little bottles, but it just did not work well.
For a 30 gallon tank I would set up 2 x 2 liters and rotate when to re-do them.
I have to agree with the others, as although i LOVE this system for a smaller tank, but it provides just enough for my 5 gallon tank.
You would need at least a couple of them for your tank, and you may as well just go with DIY 2x2 Liter pop bottles diffused through your filter, or a diffuser such as the hagen bubble ladder.
The basic concept is that yeast will produce CO2 when it eats sugar.
By mixing sugar and water in a sealed bottle with just a little bit of yeast the yeast will grow. With the right tubing the CO2 they produce can be added to the aquarium.