I used 2 of them on a 29g, but never got the Co2 high enough. I then switched to a pair of 2 liter soda bottles which gave plenty of Co2, but not consistent enough for my tastes. Then I just gave in and went with pressurized and a pH controller.
I use this system on my 5 gallon tanks ( a separate system on each tank). Instead of the ladder, I put the tubing onto a limewood airstone. I use wine yeast and baking soda in place of the activator and stabilizer. It works great in the 5 gallon tanks. I think the literature that came with the product said it works in up to a 20 gallon tank. I tried two systems on a 10 gallon tank and couldn't get them to work very well.
I add about a half a ml of the included plant fertilizer to my watering bottle (1 gallon) when I water my houseplants.