Depending on how long you've had him and how large he is, he may have had internal damage from stunted growth. If his colors were splotchy/pale it is a sign of stress. And maybe, possibly, in his small environment with smaller fish (therefore less leftover fish food) he may have had problems finding food.
Water parameters are important but in this case I think you were prob fine. It is not necessarily easy to kill common plecos and I would think that your smaller fish would accumulate toxins quicker and die first.
Hope this helps, sorry for your loss!