Water changes should be around 15%-25%, which means in a 29 gallon should be 4 to over 7 gallons. But I would think it would have gotten to the other fish first. I had a plec a while ago that just died after a little under a month. I determined it wasn't conditions because the Blue Rams (as sensitive as they are) and all of the other fish never showed the first sign of stress. I fed everything from algea wafers to cucumbers, lettuce & carrots to shinking shrimp pellots. But wouldn't eat them. He ended up starving himself to death, not because I didn't feed him, but for one reason or another, he wasn't eating what I was feeding. I don't know if something else was stressing him and causing him not to eat or what.
Another quetstion, do you have driftwood in your tank. Most plecos need driftwood in the tank....