What's causing the rise in ammonia in my fish tank? How do I fix it?

I have a 20 gal. tank, and one little goldfish and a small pleco. (Both are less than 2 in.) I've had them and the tank for about three months. Everything has been fine with the water levels until a heat wave we had here. My goldfish got really sick and layed at the bottom of my tank because the water was so hot. I did a 10% water change out of cycle to cool him down. He seemed much better the next day, but he had 'black smudge' on his side. I checked the ammonia levels yesterday, and they were really high (about 1 ppm comapared to the usual 0) Was the water change what caused the spike in ammonia? Or was it the heat? How can I keep my fishtank cooler in the future? And how do I get my ammonia levels down now?