What else is in the 40g? How is it set up? People suggesting things like L46 zebs and other Hypancistrus without knowing anything about your set up (other than the volume) seems a little foolish IMO. You say you want a pleco that is going to be a "workhorse", so I'm guessing you just want something to eat algae rather than keeping the pleco as an interesting part of an aquarium? (in which case Hypancistrus would be a terrible choice). If you've got algae problems then try reducing your feeding or lighting, maybe add some plants like pothos to help use up excess nutrients. If you want a cool fish to keep in a 40g tank then most of the above suggestions will work well if you're willing to care for them correctly.