Is it fan leaves or the cotyledons? How much are you feeding them? Mine started yellowing, i was underfeeding them. I've been so cautious of overfeeding i always go a little less then recommended and add more if needed. I was around
[email protected] when mine were that big, once they start dropping roots they can take some nutes. My clones were on 1/4 strength veg as soon as i saw root bumps and then i had to bump it to 75% strength within 2 days because they were still yellowing, i saw inprovment eithin a couple hours, 24hrs after bumping it up they all are green and have 1/2" new growth. Dn't be too afraid to increase nutes, just a little at a time, it's easy enough to dilute the res if it's too much and the plants will recover fairly quick.
My res's run around 76 in veg and 72 in flower, i haven't had any root issues, but I'm keeping my fingers crossed, we're getting into the hottest part of the year. I've added airstones because of it..and am pushing quiet a bit of air to keep the solution well aerated..i have to dial it back or i start shredding roots. I'm dealing now, but starting to look at res cooling options for down the road.