The most common leak is the O-ring on the strainer basket lid. If it has any cracks, no matter how small, it needs to be replaced. It also has to be well lubricated with silicon grease. While you're messing around in that area, check the drain plugs on the pump and strainer. You gotta remember, that pump can suck a lot. Consider how much water you can suck up a 2" pipe for comparison. Even a tiny little champagne bubble size air leak can build up a lot of air in the tank real fast.