I needed a CO2 reactor for my 180 l tank and so, as I usually do, I went down the DIY route. I had a gravel cleaner (now unused), a spare top from it's predecessor, an airline connector, some Bioballs (Pond biological filter media) and some spare 12/16 Ehiem tubing and realised these could make a good reactor Basically the CO2 is fed into the tubing just above the gravel cleaner reactor. The water flows down into the reactor where as many bioballs as can fit in were placed to mix up the water flow. All the joints on the gravel cleaner and the point where the airline connector goes in were silicone sealed to prevent leaks. Since making the unit I found that the pipe going into flexible tubing flexed and after a few months leaked very slightly. So I took the shepherds crook from my old filter and drilled a small hole. I then fitted the airline connector there and Araldited it into place. I also found it was best to fit a check valve near the reactor and another one near the regulator as water crept back down the CO2 tubing. The extra check valve solved this. This set-up ran for nearly a year before I moved the CO2 to my new tank (I will be adding CO2 back to this tank once a new regulator arrives). Interestingly my 180l tank upstairs needed just over 1bps to maintain 30ppm CO2 but on my new tank the same CO2 kit but with a glass diffuser needs nearly 3bps (in the same bubble counter) to give the same result! It may be because of the tank, but I think that the effectiveness of a reactor can't be beaten!