Leaves curling up and down, yellowing as well?

My plants were germinated from seed approximately 3 weeks ago. All of them are bursting with green and new leaves/nodes. :) I have a fan blowing above them just enough to have the least "shivering effect" as possible. I'm using an Epsom soil mix, but I don't believe this would be the problem, as the other ones are doing perfectly fine, excellent actually. The lighting fixture is a dual T8: they're approximately 2-4 inches away from it. I have the lights on a 20/4 schedule.Tap water that I sit out 2-3 days before watering has a pH of 6.5 - 7.0, as tested. Temperature is at about 75-80 degrees Fahreinheit, relative humidity is 50-60 %. When I water I let the top 3-4 cm of soil dry out before doing so. I'm going to start giving nutrients (1/4 of recommended, as people have suggested) this coming watering.

So the problem with one of my plants is that the cotyledons are turning yellow/brown, there are 4 "true leaves", as well as 4 very small beginning leaves. All of the leaves are yellow tinted. One of the two first true leaves is curling down and the other is curling up. Will my first nutrient feeding help this, or is it something else? Thank you, and if you need additional information please do ask.