There are many species of plecostomus often referred to as plecos or pleccos. There are more than a few threads on this forum with info on these fish. These fish are algae eaters and will help keep the glass clean if you have algae. They will also scavenge and eat uneaten food from the bottom of the tank (should your turtle not consume everything). Very large plecostomus have a better chance of surviving, but nothing is really safe from your turtle. If your turtle so desires, the plecostomus will become sushi lunch.
Plecostomus will not help with your water being cloudy. They will add additional bio load and magnify the small filter problem you already have. They however are great fish and much fun to watch, but you may need to feed them algae wafers if your tank isn't full of algae. My advice is if you get one.. don't spend a lot.
