Usually Petco lets you bring back the fish in a set number of days for store credit. I'm assuming the deadline for that has probably passed for the pleco...Petco will still take it, but you get nothing for him. Privately owned fish stores might pay you or give store credit, but you'd have to find one first. You can also post on Craigslist and sell it to some inexperienced noob who'll end up in the same situation as you...