You might want to give your pH a check too....I started losing any fish I added to my old 30 gal tank, including several plecos, but not the fish already there, so I checked my pH and it was <6!! Over time, tanks will slowly acidify (as well as build up nitrate), the fish in there might get used to it, but each species has it's own tolerance level. In my case the nitrates were more in line (~40 ppm, so still high but probably not lethal).
Sodium bicarbonate (baking soda) can be used to bring your pH back up, if need be. Just add it very slowly so as to not shock the fish. 1/2 teaspoon per day is safe and will only bring the pH up a few tenths. The bicarbonate is much better for your tank than any of the commercial pH buffers which contain lots of phosphates, which can cause algae problems.
Good luck! Congrats for turning to AA for help...everyone here is awesome about answering any question you could ever have!