City Water In Filling Fresh Water Tank

The filter should be on the input side or the suction side of the water pump. It's there to filter particles that may come in from your freshwater tank. If it's leaking it needs to be tightend or replaced. If the filter is on the output side it possibly allowed particles to get in your backflow valve inside the pump, therefore allowing water back into your freshwater tank. There is usually not a valve blocking city water from your pump as the backflow valve in your pump should not allow water past.