Nick,
I am sorry you lost your PBT, but the fish was healthy and eating well when it left the store. We have had that particular fish for well over 2 months. Your mother indiciated to me that your amonia was 1, which should always be zero. Also, something could have been stressing the fish.
I am sorry I cannot offer you a replacement for your fish. But there is no way of knowing what exactly happened and why the fish died. There are too many variables involved to offer replacement fish.
I know some stores in the past, have offered 2-3 day gaurantees, which can be done if you charge twice the price, but we do not. I would be more than happy to help you determine what may have happened and help you check your water.
John