Thank you so much for all your replies.
I spoke to the original pet store owner, and he agreed to re-home Mr Pleco for me.
I took him there immediatley, (this morning)
He was put into a much larger tank with one other fish.
A huge weight has been lifted, I feel he now has a chance at a better home.
I have shed a tear for Mr pleco and the suffering he mustve endured,.
I will miss him, but he deserves a better home.
I have learned a valuable lesson, to thoroughly research any species before I decide to add it to my tank.
and to not rely only on the information given to me by the store clerk or assistants.