I can't tell you exactly what caused the failure but I highly doubt it was from over tightening a bulkhead. I do know that when you modify a standard type tank so the bottom looks like Swiss cheese you can no longer rely on support from the edges only, you need to reconfigure how the base is supported typically with foam and a solid base.
Reason being is typical DIY drilling procedures result in a large number of defects around the holes that act as stress concentrations. These stress concentrations greatly reduce safety factor, even a slight surface imperfection can double the local stress. These high local stresses can now easily spread into cracks with a decent load, adding additional support to the base reduces the load and therefore the resulting stress.
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