I will tell you that I bought my aeration system from one of the companies that advertise on PB, and that it came with a pair of 4' ring diffusers.
I feed one diffuser set 12' down in 16' of water with a 1/4hp Thomas pump. It keeps my little .35ac puddle nicely turned.
I ran one shallow over the winter and it kept a quarter of the pond clear all winter long.