I use an Aqua-Medic 500 with a Maxi-Jet 600 pump in my 40 gallon tank and it works very well. It is supplied with CO2 from two 2L DIY fermenting bottles. I use a drop checker to monitor the CO2 levels. I used this system on my 135 Gallon tank for a while using 3 2L bottles and I was surprised by how well it kept up although I was just starting up the tank and the plant load was fairly low at the time. My 135 now uses the Aqua-medic 1000 with a 700 gph mag drive pump and of course the ever present drop checker.