I know this is an old thread but I noticed something that may be good information for others.
When the OP originally posted his test results he noted that his pH was 6. At that pH value all the ammonia in the aquarium would have been converted into nontoxic ammonium. This would have no effect on the pleco. Ammonia is only toxic when pH is above 7. This is when non-toxic ammonium ion is converted into the toxic ammonia. The sudden amount of water changes likely brought the pH up which then converted the ammonium into ammonia and made the problem worse (or created a whole new problem).
Most likely there was a parasite or bacterial infection that was causing the infection and a treatment with a parasite medication may have been a better course of treatment.
Regular water changes are important, but if you don't do them regularly it's best to start slowly with smaller volumes more frequently rather than a big one all at once. Fish will adapt to slow changes very well but rapid ones can be lethal.
