Have you been reorganizing them according to height so you can keep the lights equidistant from the seedlings? Do you water them all at the same time? There are two basic types of humidifiers, those that evaporate water and those that atomize it. The evaporative types have some kind of replaceable filter, atomizing units don't. If you have an evaporative type, I can't imagine anything from the city water making it to the trees. I doubt that would be a problem with an atomizing unit (what I use), but I can see where it would be possible because it puts a mist of tiny water droplets into the air. Again, I doubt this is the issue.
Presuming you have not been reorganizing trees and they actually grew in the positions shown in the picture, I would suspect it is a soil moisture issue. If you water all trees at the same time, I can see where trees further from the humidifier would dry out faster. So you may be watering trees closer to the humidifier too often. Wait until you think it is time to water them. Then compare the weight of the cells furthest from the humidifier to those closest. See if they are significantly lighter. Of course some of the lightness can come from larger tops using more water but some can come from less humidity in the air.
Too much water can impact chestnut growth. If you decide to cull one, remove the medium and look at the root. I sometimes get what I call "carrot root" were the tap root is very large in diameter and there are very few lateral roots. I presume this is some pathogen but I don't know what. It seems to be associated with overwatering.
In general your chestnuts look great! I always get some small percentage of chestnuts that are runts. Some may just be genetics, some may be me getting lazy and watering them all at the same time rather than picking up each container and checking the weight. I have noticed some issue with trees too close to a fan, but for the most part, my runts are initially distributed through my trays until I reorganize my trees by height.
Thanks,
Jack
