16" would be a better choice (maybe 18"). Here's my thinking...the bubble plum will be about 12" in diameter due to the 12" Matalla diffuser. The cross sectional area of that 12" plum would be 113 square inches. I would want the equivalent area between the diffuser and the inner wall of the pipe to allow water to flow past the diffuser. That would mean the pipe would need 226 square inches of cross sectional area. If you back that into a pipe size, you get 16" diameter pipe. Anything smaller will not be as efficient, meaning the diffuser will be acting like a restriction in it's own system.
How big around is the Matalla overall? I suspect the diaphragm is 12" and it has some extra width to it from it's frame. Let's say it's 13", that only leaves a 1/2" gap between it and the inner pipe wall. That's not much of a passage to allow 1000 plus gallons by every minute. 18" would be my "go-to" for a 12" diffuser.
I am really basing these comments on a very crude rule of thumb for fluid flows, so it may not be 100% accurate. I am very interested in the outcome, however. I cannot figure out why ponds/lakes do not have this type of system more often. I have always thought it would be the expense of the added pipe-works and anchoring systems. It would resemble an under-gravel-filter for an aquarium (without the under-gravel part).
