I would certainly queston those test results on ammonia and nitrite. Nitrite at 1ppm would be evident in the behaviour/appearance of the fish.
A cycled (which this would seem to have been) 55g full of plants should not even show an increase of ammonia or nitrite on the day following the addition of 6 tetra. What should occur--unless there is something that we don't know about your tank--is that the additional ammonia from the added fish would be grabbed by the plants. As this produces no nitrite, that's the end of it.
When was the last time you tested for ammonia and nitrite before today? I see you have the API kit, any chance it may have expired?
Just to cover all bases, have you tested your tap water for ammonia and nitrite and nitrate? And how recently?