the pics arent really clear enough to make any definitive decisions on what it could be, best way to have any chance of identifying it would be to put it in a take away container or ice cream container full of water and take photo directly from above
another way is if hes out and feeding and you can get a clear picture of him in the tank, that will give clear perspective of body ahape and patterns
i commend you on not wanting it in an inapropriate environment, usually takes people a while to work out that sometimes fish just get too big for the starter tank they buy,
i have a friend that worked in a big box store which shall remain nameless , his boss didnt care when he raised the issue of common plecos being sold to people that buy 18 inch or 24 inch starter kits .Her theory was they would fall in love with the fish and come back get convinced to buy a 55 gallon or bigger
saying it will stay small is all about perspective, a 12 inch pleco is small compared to a full grown red tail catfish,
use google when buying new fish if you cant get a definitive answer on max growth size , best tankmates or even dietry requirements of fish, some eat nothing but vegetable matter and high protien diets will cause bloat or worse, then theres omnivores and the carnivores