I wouldn't do anything with the pH, now if she got fat right before she died, I got a few questions for you. First did it look like her eyes were bulged out a little bit? also If you looked at her from above or as she was swimming away did it look like her scales were pushed out, kinda like the scales of a pine cone? If you had her for two weeks, and she was active, I'm willing to say it wasn't the pH. Convicts are one of the toughest fish around and can take almost anything, I heard a really accurate saying that to breed convicts all you had to do was add water and then add fish. Here's a couple more questions, How long has the tank been set up, and have you checked your Ammonia and Nitrite levels?