Fletch is right. Most plecos you see in the stores don't stay small for long, and if they are just labeled as a small pleco, then it's probably a common pleco which can get 18 inches long! Look for plecos labeled as bristlenose plecos and if your tank is 30 gallons, you could probably get 2 of them. If you tank is 20 gallons, just get one. Most bristlenose plecos will get around 6 inches. The bigger ones get very clumsy as they get bigger and will terrorize a smaller tank. I have one over a foot long, and he's in a 100 gal. tank. Even in that size tank, he can make a mess and tear up the tank! A good place to research different types of plecos is . the catfish of the month articles are great reads, and will probably give you some great ideas on some plecos for your tank.