Since you have tetras in the tank, I second the PWC
to keep the ammnia less than 1ppm, preferably even less than 0.5 ppm
. Yes, plants do take up ammonia, but till they do, your tetras are being exposed.
Oh, and nitrite is NO2
? Yep, once you have enough bacteria to handle the ammonia production of the fish, your ammonia will be zero, nitrite zero, and unless heavily planted, nitrate will have increased.