Tetra Optimat....possibly the most pointless CO2 system ever, absolutely no way of keeping CO2 stable as you manually squirt CO2 into a 'diffuser' cylinder and it enters the water at its own speed - once the 'diffuser' is empty you need to add some more - not easy if you are out! I gave up with it, DIY and a ceramic diffuser is a better bet, pressurised even better.