You only feed the plecos two times a day with the wafers, and generally one wafer at a time or no more than they can eat in two hours. So says my package of wafers I feed to my plecos.
About the death - did you check the water for ammonia, ph, etc? I did have one pleco die, but he or she, was being constantly harassed by the two oscars - they kept nipping at his middle back and there was a yellowish lump that developed. I took everyone out of the tank and cleaned it all, changed rock, installed a filter bed, and quarantined the other fish until I felt it was clear to put them all together again.
Here are some pages on plecos -
Robyn's Pleco Page Plecos - Aquarium Forum Aquarium Information - Alternative Fishkeeping Plecostomus, Pleco - Hypostomus plecostomus
This article on freshwater fish diseases:
Freshwater Fish Disease