People are going to tell you nitrite is toxic to fish and that you shouldnt add them until all perameters are perfect. I am having the same problem, ammonia is zero and nitrite is in the caution, (.5)
I have three cories in this mess with four tetras and all are fine except one cory that had a problem with his barbels before I moved him over to this tank.
Good luck. If I could do it over, I would of waited. Keep doing your water changes until everything is zero. I am doing 50 percent WC twice a day and it is getting better fast.