I agree with Brookester... You don't need one, and most plecos get huge anyways. It would quickly outgrow your tank and add to the bioload. It also sounds like your tank is already overstocked, so adding a pleco would make matters worse.
Here's an excerpt from an article about plecos.
"Two months ago, you bought that little 10 gallon aquarium and stocked it with the usual nice, friendly community fish. Everything has been going really well. You worked through the cycle, you lost one or two, but that was to be expected. After the tank cycled, you added some new fish, and they’re doing great. But. . . what’s that? There’s an ugly brownish spot on the front. The tank is rusting! Quickly, you call your fish friend and ask what’s going on with the tank. “It’s just allergies? What do you mean my tank has allergies?! Oh, algae! Okay, so I need an algae eater!”
A visit to the fish store reveals an entire world of allergy eaters-er, algae eaters. The salesperson steers you, oddly enough, away from the fish labeled “algae eaters,” explaining that they get kind of big, don’t eat algae when they get big, and can kill your fish. Instead, he recommends this sucker called a “Plecostomus.” You plop down your $5; you and Mr. Pleco are on your way home.
Mr. Pleco does a great job of eating the algae in the tank, too. And two weeks later, he’s dead. If you’ve got a good store, they explain to you that you do actually need to feed your pleco, sell you another and a bag of algae wafers. If you’ve got a bad store, they just sell you another. So, you and Mr. Pleco (the Second) head home, and you feed him. And he lives. And he grows. And grows. And grows. And grows. And grows.
The fish in question is called the common pleco, and is known scientifically as Hypostomus plecostomus. About a half dozen other fishes are also commonly sold as the pleco, and ironically, the most common around here seems to be Pterygoplichthys padalis). They all look basically the same, and can be pretty hard to tell apart. They’re members of a family of catfishes known as the Loricariidae, which has somewhere around 700 described species and something like a billion undescribed species. While the common plecos are kind of ugly, some are breathtakingly beautiful (and some are really ugly).
As many, many aquarists have discovered, one of the big problems with the common plecos is that they get big. Really big. These guys tend to max out, in the average home aquarium, at between 12 and 24 inches, and some can get bigger. This puts the fish at a size that is far too large for average home aquariums. As these are very active fish, coming from fast flowing, heavily oxygenated waters, their life in a small aquarium with decreasing capacity (as a single large fish becomes an increasing burden on the tank) is undoubtedly torturous. Many aquarists argue that they just “lay there” anyhow, but I doubt very much so that those aquarists are ever looking at their tank under dim light at 3 a.m. Plecos, like a lot of catfish, are kinda sorta nocturnal.
So, what’s an aquarist to do with an overgrown catfish that can wipe both sides of the glass clear with a swipe of his tail? Take it back to the fish store, you say? You’ll be lucky if they’ll take it. Unfortunately, you and 10 other people have been in today with an overgrown pleco, and – surprise surprise – there’s no market for them."
