Pleco Not Pooping

I have a 50 Gallon tank and I have treated it twice for parasites due to my Common Pleco Pooping white empty strands of poop (about 2 weeks ago) after the treatment he started eating however he hasnt pooped and it looks almost like he has a parasite still stuck in his butt (it's like a little worm). I dont know what to do or how to treat him. I have him 3 balas and 2 silver dollars. All seem fine but we all know a Pleco is pretty much a non-stop pooper and he hasn;t been pooping at all. Oh and all Levels Nitrites Nitrates and Ammonia are fine