Either that, or it's leaking out from the very bottom. When the water gets deep enough it forces the water out, but when the water level drops the pressure is reduced and the leak closes back up again.

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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).