I agree 30 is quite a high temp for neons, trying lowering it by a degree or 2 a day until its down to 24-26degree's warm

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Are the neons flicking or rubbing themselves at all on stuff in the tank? Are they gasping/rapidly breathing and are they hanging out near the top or bottom of the tank a great deal or showing any other odd symptoms whether behavioral or physical etc? Are any of them looking a bit pale at all/is their colouring looking normal on all of them?
How planted is the tank? In very planted tanks with lots of healthy plants, the nitrates can be as low as 0 since plants absorb nitrates as a fertliser, however if the tank isn't especially planted or has no live planting at all then something is most likely amiss as nitrates at the end product of the ammonia and nitrite process, and if there isn't any nitrates in the tank then it means there is probably something wrong with the stability of the water quality in the tank etc.