Growth stunting from living in too small of tanks.
Not cycling a tank.
Not feeding enough vegetables.
Sometimes they consume rather odd objects, and most of the time spit it out. Rocks also may get stuck in there throat and kill them. Stunts in growth from smaller tanks is also a big factor, but they shouldn't grow from the size of the tank. I have never really seen a fish out grow a tank. It depends on the tank size.
tank not cycled, too small of a tank, the plecostomus needs at least a 55 or more gallon, water too dirty, temperature too low.
Starvation they must eat lots of algae and veggies.
they don't have algae in there tank
