Like he said, pump info would help greatly! Have the diffusers been cleaned in that time period? Some systems usually have an air filter (muffler) that need changing/cleaning (changing is easier) every 4-6 months. Have they been maintained?

Is there a pressure gauge on the system? If not, think about adding one to help diagnose the problem in the future.

BTW, welcome to the forum!!

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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).