If I understand you saying air in the water when the pump runs then you have a leak at a fitting or cracked pipe that lets a little air in.
Mine comes up out of the top of the tank, bends over and goes through the floor then back up by the pump.
I would start at the pump and work back to the tank looking for somewhere it could be air could be coming in. Any loose fittings, cracks in the filter or back-flow valve, pump housing? Stress marks in the pipe, wet spots where tiny leaks have occurred?
If you still can't find it try filling the tank all the way up to get some pressure on a hidden spot to make it leak water out when the pump is off.
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