Can you post a pic? how is he acting otherwise?
How much and how often are the water changes, additives used, filtration and is the tank cycled, live plants? other tank mates, do you have any water pram numbers for ammonia, nitrite, nitrate, pH
What kind of pleco?-the common type can get over 2ft and their diet changes to sucking slime off slow moving fish and the smaller type can get 3-6in depending on which one you get and IMO/E the smallest tank would be 20g and 30g is better