Common Pleco's tend to be to hardy for their own good. They can survive some horrific conditions that cause them to become stunted. A condition which compresses their internal organs. Unfortunately the affects of stunting don't usually show until the damage is done and their life is shortened by several years. The stunting is also painful for them.
So these terrific fish with unending personality end up living short painful lives, due to the ignorance of many a fish store employee.
Absolutangel104 is correct about the 125g tank for a Common Pleco. Many fish sites do say 90g, but that is an absolute minimum. All you have to do is watch an adult CP swimming and you'll see why the 90g just isn't long enough for them.
But in a 125g+ tank, they are awesome to watch swimming, so strong and graceful.