Looks like you've done some good work! One thought...Your back pressure is 1.5 and the relief pops at 4. With those numbers you will not be able to place a diffuser in 7ft of water. 1.5 + 3.5 (7ft) = 5. Your relief pops at 4 so the deepest you can go is about 5 feet. Anything you can do to reduce the back pressure? The 100ft of 1/2 inch line is almost no back pressure. I suspect the majority of that 1.5 is coming from the diffusers. Maybe lots more holes in your diffusers?
……..Just a thought
You're right. I thought about this too. On the other hand I tested it using twice the length of tubing. I'll see what it does in the pond, and then I might add another row of needle holes if I need to. ... Or maybe I'll just do that first, it couldn't hurt.
