How to control nitrate levels.?

I have a 10 gallon tank and I had a fish die yesterda morning. I removed the dead fish, did a water change a little more than 25 %. I added some prime and the clorine reducer. I tested my nitrite/nitrate levels last night. Nitrite is fine at 0 but nitrate is at 80. I also tested my ammonia levels last night and they were fine. I tested my nitrite and nitrate levels this morning and the same thing. How can I bring down my nitrate levels, why is everything else okay and this not?