It usually takes more than three weeks to establish the nitrogen cycle, sometimes more than eight weeks. Even after the nitrogen cycle has been initially established, it can take months for a new tank to be fully stable.
Stress Coat is a
water dechlorinator that also neutralizes heavy metals. In addition to providing these same functions, SeaChem Prime claims to detoxify
ammonia and
nitrite form 48 hours. That claim has been disputed so I would not rely on it but I would use Prime in the hope that it detoxifies nitrites.
There is some debate about how harmful low levels of nitrites are. There is wide spread agreement that any nitrites are harmful, but low levels below 1
ppm may not be tragically harmful for short periods of time. Some people will say that 2 ppm nitrites requires emergency action. I am not convinced that is so. You definitely want to get nitrites down, but I am not sure you need to panic.
I would do whatever
water changes are necessary to keep nitrites below 1 ppm. A 50% water change would reduce 2 ppm nitrites to 1 ppm. You may need to do 50% water changes daily for a while.
50% water changes are large and can be stressful if not done correctly. It is important to use water that has been dechlorinated and is similar temperature to tank water.
If you have not done large water changes in the past, you may want to do multiple smaller water changes to make the change more gentle and develop your water change process.
You can also reduce feeding until you get nitrites under control. Less food, less fish poop, less ammonia, less nitrite.