Pond Boss Forums AERATION Systems questions Existing aerator ? And upgrade ?

I don't think you want open water other than a small hole depending on your Biological O2 demand.. with LMB and CC you run a risk of super-cooling the water below that 39-40 deg point which will (could) cold-stress your fish.
2 diffusers would require 1.5cfm each at the diffuser but over-size slightly. something capable of 5cfm range at the pump would be good. Loss between pump and diffusers are affected by hose size, length and depth of diffusers.