Too hard for me to read the small chart, but it looks like the 9" is designed to properly operate at anything between 1.5 and 3 cfm air flow. So all you have to do is match your pump output to your air flow.
Your pump is rated at 4 cfm but it does not say at what pressure. Lets say, just for example purposes, if the pump is 4cfm at 0 psi but at a 10' depth (5 psi back pressure) it is 3 cfm. Then at a ten ft depth you could use a single 9". Or if it is say 3.5 cfm for the pump output at the depth you want to run it, you could either use 2 - 9" diffusers or a single 12".
You just have to know what your pump output is at the depth it will be running (roughly .5 psi for every foot of water depth) and match the diffuser(s) to what air you have available. Another way if you have too much air would be to have a bleed off valve and waste some of the air but that is not preferred because you are just burning electricity to waste pumped air. I would rather run it through another or a bigger diffuser.
If your pond is circular and a single diffuser would work well, a single 12" might be fine. But if it is kidney shape or some other irregular shape with deep points at different places, the two 9" diffusers in separate places might give better water turn over.
Remember, the purpose of the air is not to aerate the water with the bubbles, although it will do a little of that. The purpose of the air is to create a water flow from the bottom up to bring bottom water to the top where the air-water interface is what is causing the water to take on more air. You are actually pumping water with the air and it is the water movement that you want.
If those charts that I can't read say it is alright for a certain air flow through the membrane at a certain depth, then go by what is on the chart. You need to know the depth you will be placing the diffuser, and how much cfm your pump puts out at that depth. Then match the diffuser sizing according to what cfm they require/handle at that depth.
If you put too much air (more than it is designed for) through a diffuser it likely will shorten its life by ballooning it up and stretching the membrane more than it is designed to be stretched.
Last edited by snrub; 01/11/16 04:29 PM.
