Did you notice anything odd about the plecs? Like their bellys, did they look sunken in or anything? Also how are you acclimating them, I know plecs are pretty hardy critters, but a major change in pH, hardness, or other water parameters could harm them. You may try to drip acclimate the next one, or at least test the water he came in to see if it is a close match to yours. (accuracy won't really matter, so you could just use strips, you just want to see if they match) Other than that, I don't know what to tell you...