Could be that when filling the tanks you 'entrained' a lot of air in the water causing a great amount of micro-air bubbles to become 'mixed' in the water. These bubbles sometimes take a long time to 'effervesce' out of the tank water.
Try filling the tank 'gently' so as not to mix-up a lot of air into the water.
Other possibilities: 1. leaking seal / o-ring on the 'suction side' of the pump. 2. pin hole or loose connection on the 'suction side' of the pump piping. 3. pin hole in the pump diaphragm or developed air leak in the pump gasketing, etc..
Best way to 'test' for the above .... blind off the suction side of the pump/piping, (thumb usually works), turn on pump to develop max. vacuum, shut down the pump and see how long it takes for the vacuum to 'decay'.