This is going to sound strange, but I promise you it will definatly work. I used to run into this problem frequently with my clients. Do this exactly and all will be good in a week!:
Turn off the light. Cover the tank with a blanket. Do not feed. Do not expose the tank to any light at all for 3 full days. On day 4 remove blanket, turn on light as normal, and do a 50% water change vacuuming the gravel. Another 50% on day 5.
I guarantee you this works. I have been asked this question 100 times by people who are frustrated with chemicals. Give it a go! Feel free to email me any questions because i know this sound wierd.