stringy feces

Hi, I have a 12 inch Rivulatus in a 90 gallon tank with 2 plecos. The tank is being filtered by 2 Aquaclear 110s. I'm using a 250 watt aqueon pro heater set to about 78° Fahrenheit. Lots of big water changes and I'm currantly feeding (or at least trying to feed) Fluval bug bites pleco and...

I have my albino oscar with some stringy poop. I had already put him in a quarantine tank before seeing the poop due to being picked on by another fish (or so that's what it seemed, from nipped fins). Oscar is about 2.5". In a 75 gallon (48x18), with a JD, 5 silver dollars, and 5 S. juruparis...