Browning of leaves on Northern Lights Seedling

Not sure how to close this questions either. Thanks for the help.

The cup contained 100% Fox Farms Happy Frog. No perlite or anything. Both cups were filled in the same fashion and with the same soil. I think next time I'll add some perlite because why not? So, I poked around her root base a little bit and beyond the initial root going into the soil, her roots are looking the same color as the soil after I get past the initial white just under the soil. Compared to my GSC where I can am able to see much more white further into the soil. Is this is root rot, do I need to replace the whole soil bag? Or can I just remove her and let the bag dry out? At this point, she is a good week behind GSC so I am thinking she is going to be hard to save at this point.

Drip something on it? Maybe, soil is too hot for that particular plant. as long as you wait for top layer to dry before irrigating again.. should be fine in that regard (small cup, so i'd go 1/4" to 1/2", but 1" deep in a pot with more mature plant) Always get a little runoff to ensure it is fully saturated... repeated dry pockets wreak havoc over time - both pH and nutrient concentrations issues possible. i'd use a bit more perlite in future. overwatering should only occur if you run water through your substrate for 15-20minutes or more at a time... a good 50/50 mix with perlite avoids drowning roots. 70/30 in coco. may have some sort of root issue. gently pull out of cup as if transplanting and see if roots are brownish or slimy... if so, gently replace and add some h2o2 to an irrigation. 1/8th dilution of a typical 3% bottle is safe.