I was doing a water change today, and found my pleco dead at the back of the tank. I worked today, but I know he was alive last night, so he's died in the last 12 hours. All the other fish seem fine, and I just got my nitrates down to a level of 10 ppm. I had a high nitrate problem, so I've been doing water changes every second day to lower it. Could the water changes (about 30-40% every two days) have been too stressful on him? Oh, the PH is 7.2, I just checked it. Also, the tank is not planted, and is 20 gallons. In it are some neon tetras, 2 strawberry tetras, 2 platy's, 2 serpae tetras, a cory, and two swordtails. All the fish are quite small. Thank you.