Yellow spots and dry seedlings

Curious what things you have tried to help avoid this situation?? all the plants look like they are a little lemon lime green to me vs green green. It could just be the camera though. what is the medium composed of under the pebbles/LECA? what are you feeding them and how often? Do you use any air-stones in your RDWC reservoir? Do you use a EC pen to calculate feed concentrations? Have you monitored the PH of the individual container mediums? Could be Mg like GuiltyPassion said. or something else, a lot of root issues mimic nutritional deficiencies/abundances. best of luck

65%RH? where is the monitor in the tent? top bottom 65% at the canopy of such a small plant would be insanely high for a plant with leaves, high humidity is only useful until the seedling has first set of leaves then humidity should be dropped immediately, once a seedling has developed a root system it will produce its first set of leaves, the high humidity is useful to allow the young seedling to manipulate soil and break top layers of soil, but anything over 60RH% and the leaves will start to close their stomata on back of leaves as the air is too thick with moisture essentially "drowning" the tiny plant's respiratory system this is in response to high RH, this breakdown in "breathing" grinds photosynthesis to a halt, no photosynthesis, everything grinds to a halt, I'd guess your plant is on strike until you reduce humidity to comfortable levels of 40-50%rh depending on the strain.

If you have a look through the pictures and match it’s to the Signs of Magnesium deficiency. ( I think it’s that). https://2fast4buds.com/news/different-types-of-cannabis-plant-nutrient-deficiencies

The ph stayings at 6 constantly (blue lab controller) in RDWC with drippers. 20k Lux. Humidifier water is RO. Ppm 300 water temp 20c. Tent temp 25c and humidity 65%. I thought it was the hard water being used in humidifier so this grow I’m using RO in it and it’s still happening.