A lot of people don't realize, when they buy a pleco, just exactly what it is that they are buying. You know that you have an algae problem in your tank, so you go to the LFS to get something that will eat the stuff for you, and 90% of the time, a pleco is recommended. Plecos do a great job of eating algae, but when they get it under control....then what do they eat? It is good that you are feeding the supplemental algae wafers, a lot of people don't do this. I just had a pleco die that was "rescued" from someone that had no idea what they were doing. Looked like it came from a concentration camp, it was so thin. A 2-week regimen of frozen brine shrimp, frozen bloodworm, flake food, freeze dried tubifex, shrimp pellets, dried marine algae sheets, and algae wafers....the poor guy was finally filling out a little through the stomach, but the damage was already done, he never did regain body mass.
I had a pleco live under my care for 12 years, don't know how old he was when I got him. Ended up being about 18 inches long. Do most people think about the eventual size and tank requirements of these fish when they buy them? Are you prepared to house a fish 18 inches long? Do you know how much water they can splash out of their tank with their tails?
My opinion is that you are OK with the pleco in your 20 "temporarily", but you should really be making plans to get him into something bigger very soon. To get the most enjoyment out of this hobby, you need to do what is best for the fish....a fish that has it's basic needs met will always be better looking than one that is just "surviving".