If all the water in the tank was fresh dechlorinated tap water I would be confident it was the ammonia from the chloramine which was bonded to chlorine but I do not know what your big tank water has for ammonia. I would dump the water and fill with fresh dechlorinated tap water. You big tank's water has no real bacteria benefit. You could wait until you have one of the treatments and treat your tank before adding the fry. I have had fry in fry nets for a couple of weeks before transferring them or take your chances and put them in now.