Ok so this is what you need to do:
1) DO NOT STOP OR SLOW WATER CHANGES. Yeah, you might not have had this problem, but that is not the right solution. Fish need to be able to live in clean water, and water changes are how that happens.
2) Test your tank water and tap water for BOTH nitrates and phosphate. Post the results and that will give some insight here. Yes, nitrates in tap, it happens to a lot of people. I had a friend in arkansas who was getting 8-15ppm of nitrates in his tap!
3) Reduce/eliminate lighting for a while and see if that helps out. If you have plants and can't eliminate lighting for a while, just reduce it to 5 or 6 hours a day, tops.