pH won't be your biggest problem either, it will be the ammonia, nitrites and nitrates that will give you an idea of what is going on. Did you get test kits for these? The Nitrates and ammonia in particular.
Things would have gone well until the toxins built up over time - around the 2 - 3 week level which is why things were OK to start with. Whoever advised a 24 cycle time was gravely mistaken, as you have found.
African cichlids in the wild live in a pH of around 8.0 plus. Mine live in 7.0 as that is the pH the breeder bred them in and they have never known any different. They are living and breeding happily at that pH, it is not critical. This also saves me having to try and alter the pH as my tap water is 7.0.
I doubt any treatment given the plec would have triggered this off, just coincidence (assuming you followed the instructions).
Wait until the tank has stabilised then give us a rundown on what fish you have left and we can check stocking levels then.
Good luck! 8)