Ii have the same problem. As Jayhawk said, regardless of how little water you fill your tank, once you are underway, the water pressure will force water out. It is a poor design issue, the crack is usually in the 90 degree angle in the front corner and possibly hard to see. Also, the tank is not supported enough to prevent the flexing due to the weight of the water with no baffles.
I have tried many solutions, all eventually failed due to the amount of pressure outward. The best solution if you are not willing to change the tank ($1500), is to drill a whole between the stairs wall and the bulge under the stairs to allow for the water to drain into the area where there is a bulge pump. Although I do recommend dumping a capful of Clorox every week.