My Piranah Is Acting Weird

My water parameters were poor so i did a %50 water changes and that did not lower the nitrate leveles so i did a %100 water change and added new gravel. My piranah seemed fine for a few days now he is floating in the upper right corner of the tank next to the heater. He won’t eat and sometimes he randomly farts around thrashing everywhere and bumping his head onto the glass. I have a 20 gallon tank with 1 redbellly and the temperature is 80 degrees. Water parameters are acceptable. Does anyone have any idea of what’s going on?