I set mine up around 6psi. But it's all about your ALK level. I set my drip rate fast like a stream not drop so it will clog up less then I adjust the bubble per minute accordingly to what ph inside the reactor: example ph 6.7 will give you 8dkh now if you want higher ALK then adjust faster bubble per minute or if you want lower ALK then slower bubble per minute. The black knob is to adjust the bubble size which you can use to fine tune the ALK level too. Bigger bubble which mean higher ALK smaller bubble is less ALK. Just watch your ph inside the reactor and adjust. The higher the Ph then less ALK the lower the Ph then more ALK.