Still might want to place a tee in the line near the pond. Can add a blow off and/or access valve and a gauge.
If air volume at the bubblers decreases but your gauge pressure is the same, trouble is downstream of the gauge.
If air volume at the bubblers decreases and your gauge pressure is lower, perhaps time to rebuild your compressor.
Troubleshooting pipelines is much easier with more info! esshup, may chime in if adding a tee is a bad idea. (I work on much different pipeline systems.)
