Fresh water tanks all have vents. If water is used out of the tank, then it is replaced with air. If you are filling the tanks, the air is replaced with water. Problem is, some tanks have vent hoses that siphon water from the tank if they are not positioned correctly. If you overfill your tank, it will drain out through the vent hose.
I hope you didn't crack your tank by pressure filling it over its limits. It happens. Fill it slowly and stay there with it while filling.
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