well in that case it would be easier to help you if you gave the acurate results. traces of nitrites or ammonia would tell a story and almost certainly give you a reason why. as cyclops says its pretty certain to be a mini cycle . im not knocking any one elses methods on this site but ive heard some posters here recomment 80% water changes!!! and to be honest if you've done a change like that a mini cycle is very likely
It could be from a certain type of bacteria, at your local pet store they should have something that will clear it up. The one I use is called "B Clear Biological Water Clearer"
most likely your beneficial bacteria in tank have casualties and the tank is recycling, maybe you have don a big water change or clean most of your filter media