If your pleco is exhibiting any of the following behaviors, they may be experiencing stress: loss of appetite, hiding in a corner or by an object for extended periods of time, flashing (rapidly swimming back and forth), aggressive behavior towards other fish in the tank, rapid breathing, and/or poor coloration. Additionally, you should pay attention to changes in their normal behavior such as reduced activity levels or becoming more reclusive than usual. If you suspect your pleco is stressed then it's important to take steps to reduce stressors in their environment such as improving water quality parameters or providing them with a larger aquarium.