Need help! I've recently bought a water testing kit and begun testing my 4 year-old aquarium's water. pH turned out alright, ammonia and NITRITE all sit nicely at 0 ppm, but my NITRATE (NO3-) levels are at a lousy 160 ppm!!! I did a major water change immediately, and added the nitrate detoxifier, but the levels remain the same after a whole week!!! I just did ANOTHER water change this morning (~ 10%), but the levels remain the same!!! Any suggestions? THanks