This is certainly a good place to post it

Do you have any fish on your tank?
Having plants without fish on a new tank (a week to two) will definitely show no ammonia or nitrites, but it would spike once you add the first fish.
The reason is that ammonia, and later on nitrites and nitrates are created from biological waste.
Fish create it on a daily basis...
Plants are creating it when they start decaying, and on a much slower rate then fish (depends on the quantities of each of course, but for aquariums it's usually right).
In addition plants are also using nitrogen elements as nutrition, so the little nitrogen elements created by some can easily be consumed by others.
In such a case you have a "mini-cycle".
BTW, I love black neons, but the regular neon tetras or cardinals could do great on such a tank.