hey liz, sorry i can't help out with your pleco , but you have id this leco as a common , you have answered one of my many questions, sometimes i see plecos just under a foot long traded in at the lfs and they sometimes have that deep black color. i have always wondered if these were commons.
So i thank you so very much liz
My opinion is that he could just be a liitle sick, when the lfs here gets traded in big commons they stick them in a 90 with four oscars, overstocked yes but big fish sale slowly, oscars are fully grown, and that tank stays above 80f maxing out at 84f , they eat those oscar pellets from hikari, basic homemade flakes and spirulina flakes, what i find remarkable about this lfs is that there large fish in this 90 are so active, and one of the oscars that came in with hith is healing up and the oscars have also startecd to breed.
I recomend getting spirulina .