Seedling yellowing.

Yeah donno about that 6.2 lockout nonsense, but you should be feeding closer the 5.8 at this stage regardless, and you def should be able to bump the nutes up a bit, just don't go crazy

6.2 will not lock your plants out in hydro BWAHAHAHAHA what nonsense... 8 is too high though and you have fixed that.

This is easy. It's not a variety of things. It is your ph. Growing in Hydro is not like growing in soil. Ph is much lower. I run a 5.8 veg and a 6.0 to 6.1 flowering. 6.2 and over is lock out. Under 5.4 is lock out. If the ph is not right the roots don't work right. .1 ph is a big difference in Hydro, and what nutrition is available for plant to access. So that's what is going on from what I see.

It could be a variety of things. Check the distance and amount of light your seedling is getting. Try to get the humidity around 70% and flush it with a few gallons of PH'd water around 6.8-7 and just let it recover. Once in veg, you can give it some Nitrogen as well. 80 is perfectly fine and I wouldn't add nutes as your plants should be fine with the nutes in the soil until around day 35-45.

Need more info...Your medium is "Sponge"? What the hell is that? Whats your humidity? What are you feeding it, and how often? What is your watering schedule? Whats the lighting you have?