Same thing happens to me. My tank has the inlet at the top from the well pump, and the outlet to the house pump. I would fill the tank and it would be almost empty in a day. I'm guessing your water is leaking out back into the supply line from your tank as the inlet is at the bottom? One toilet not used and turned off waiting on inner replacement. the other no obvious leak but my one sink was leaking inside the wall. A man came out recently and fixed that leak. One other thing. I would hear the house pump cycling for a second or two, which meant I did have that leak. Now it doesn't cycle after the wall leak was fixed, but the tank still lowers daily. The only way it doesn't is if the tank valve between the tank and house pump (what you want to install) is turned off. Very strange, as i don't see anywhere the water is going like it did with the wall leak from the bathroom sink.
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