As I told you yesterday, and we told you 3 weeks ago, your tank is cycling. You can continue to look for some other answer, but you're not going to find it. Your tank is cycling, proven by the fact that you are doing massive water changes and the nitrate levels stay the same.
When the tank cycles your Ammonia goes up, as it drops, the nitrites rise, as they drop (which is where you're at right now) your nitrates rise. when the Nitrites and Ammonia completely disappear (to zero), and you only have Nitrates you try to get those as close to zero as possible.
A completely cycled tank has zero Ammonia and zero Nitrites. You have a low reading on your Nitrites (as stated yesterday).. You're cycling.
I cannot say it any clearer than that.