cephoto wrote:
I wouldn't think a p trap plug would make the tank gauge register 3/4 full and yet VERY little coming out of the outlet hose, go down to 1/4 over night(no further), go back to full quickly, and not drain below 3/4 unless left over night again. I think the gauge is ok and besides, the water will run freely down the sink till the tank registers "Full". As others have said, more likely a mass, something in the tank not allowing it to empty except verrrrry slowly.
VENT FIRST
Then if that doesn't clear it ....poke up the drain line