-What are your parameters?(Ammonia, Nitrites, Nitrates, Temp, pH).
-Did you supplement there feedings with Algae wafers and fresh veggies?
-What were the other Inhabitants of this tank?
-Were there any fish larger than them in there that may have killed them off?
-What species of plecostomus?
-Was there a high Oxygen level?(consisting of Air Stones or Powerheads).
-Water change percentage and how often?
Now explanation of why I ask.
-Now parameters are important in any case because they give you the main answer(most times). It may have been high ammonia or nitrites or nitrates. Most plecostomus like temperatures between 70-80* F and around a 6-7 pH.
-Some plecostomus are carnivorus(but I doubt this is the case until we know what species), but the herbivourous plecostomus should be supplemented with Algae Wafers or fresh veggies.
-The reason I ask what other inhabitants is because they may have died from stress if there were a lot of aggresive fish that found the plecostomus threatening.
-If there was any larger fish then they may have picked on them and stressed them out.
-This can help us determine if they had the right dietary needs!
-Most(I think all) like high Oxygen levels.
-Plecostomus are messy and sometimes large fish that need water changes, IMO, twice weekly. If done weekly I would say 30% minimum for water change. You should do water changes weekly!