Many of us here understand and know a lot about fish keeping, others come here because they haven't had an opportunity to learn. Many LFS's are in this business strictly to make money, and accurate information doesn't always do that. Unfortunately, too many people AND animals fall victim to this, which is why places like FF are here.
Some legitimate points have been made in this post, and now that accurate information is available, I'm sure everyone will do what is needed for the animals.
Some basic information on the common pleco:
Full grown, they reach about 18 - 20 inches long, which also means 4 - 5 inches wide. The common pleco is actually a catfish, and retains some of the catfish habits during it's life, such as scavenging for food along the bottom, preferring to feed nocturnally, and needing caves to hide in. The common pleco is primarily a vegetarian fish, but not completely, so they qualify as an omnivore. When there isn't enough food source available, they have been known to feed on other fish in the tank, or when they are feeling too crowded they can become sllightly aggressive towards tankmates. Common plecos are large waste producers, so water quality can become an issue in a smaller tank as they grow.
High nitrate levels over a period of time will stunt growth, cause internal damage, and eventually lead to an early death for the fish. 1 common pleco should not be kept in a tank of less than 75 gallons for space reasons, and so they are able to find enough food to keep them healthy and well fed. Substitue foods can be added to their diet, but the common pleco is also known to be easily spoiled, so if feeding extra foods, you'll want to watch the intake and maintenance levels the fish is performing in the tank. If fed enough on other foods, the common pleco will usually stop eating algae from rocks and glass in the tank (which is why most people add them to a tank in the first place).
The common pleco can withstand a wide variety of temperatures in their environment, provided the temp is stable. This makes them desirable for keeping in outdoor ponds during warmer months.
It would be helpful if you could post water params for ammonia, nitrite, nitrate, and pH levels. If your pleco has pretty much stopped growing at 7 inches after 2 years, there is a reason for it. It could be that you don't have a common pleco, it may be another species, or it could be water quality and food related, or it could be a genetic problem (though this is highly unlikely).
If you can provide us with more information on the tank, then we can help you NICELY, POLITELY, and in as much detail as possible.
