No fish "grows to the size of its tank." This is a myth. The fish become stunted and ill and die prematurely. A pleco will grow to two feet, and can outgrow a small tank very quickly.
As for goldfish, they have a lifespan of 20-30 years (and I've heard of Koi carp living much longer.) Goldfish forced to live and become stunted in little bowls or small tanks die within a couple years more often than not, at only a fraction of their normal lifespan.
I know sorry that was my bad :P I actually know quite a bit about goldfish and koi, I miss spoke :P, I was talking about stunted growth and was asking because maybe her plecos were getting stunted by the tank they were in :P
How big do plecos grow to be? I've heard different things about growing based on the size of their tank (like a goldfish) vs. growing without abandon.
My family had a pleco that lived for about 8 years. When we got it at the pet store, we were told that it wouldn't grow much larger (it was under two inches). If I recall correctly, it ended up growing to be about 6-7 inches in a 30G-ish tank. Knowing this, I wouldn't buy another one unless I had a lot more space. They are kind of boring and poop a lot. They do, however, seem to live a long time...