You only need one gauge for the whole system, not for each diffuser. The 1/2" ID lines will work, typically the self sink lines are between 1/2" and 5/8" ID. You can put the gauge on the manifold at the edge of the pond, or if you have a manifold near the compressor you can put it there. Same with the single pop-off valve, it can be mounted anywhere. I'd get one that opens between 15 psi to 25 psi maximum.

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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).