Yep, .5 is a ballpark figure. Better to overestimate the pressure than underestimate the pressure. More wiggle room so to speak.

Ballpark: 5/8" I.D. up to 300' run, 3/4" 300'-500', then go to 1".

What is the CFM rating on the double Matala 9"? That will give you an idea of volume.

Do you have a pressure gauge on the outlet side of your compressor? If not, plumb one in. It is a great diagnostic too., and a way to monitor the diffuser performance. I'd get ont that reads 0-15 or 0-25/30.

For a learning experience, take the air hose that you are using, disconnect it from the diffuser and float it on the surface. Read the gauge. Place it in the bottom of the pond. Read the gauge. Connect it up to the diffuser, take another reading. Verify that their specs are correct. If so, you should only see 1/4 pound change in the pressure gauge. I'm interested in what readings you see on the gauge.

Your straw anology is good, but the volume of air has to be considered. Air could be considered a fluid, but just not as dense.

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