Did you test for ammonia as well? *How often are you changing water, and how much? *The fact that you have nitrite in there at all is not normal and would indicate that all is not well with your cycle. *Your nitrate readings are getting high as well. *Also, I would not trust test strip results...too many people have had issues with them providing significantly inaccurate results.
I would suggest doing the following:
1) Do a 50% water change as soon as you can. *Vacume your gravel while you are at it. *Many, many problems are sooner or later going to trace back to water quality, so this will buy you
* *some time and address the most likely culprit.
2) Wait a few hours after the water change and then re-test your water parameters, if at all possible using a liquid test kit. *
* *This is a good one, widely used:
http://www.drsfostersmith.com/Produc...&N=2004+114130
3) Be sure to test for ammonia as well as nitrite and nitrate.
Let us know the results, and hopefully we can provide some possible answers for you.