N deficiency i think

ok this plant is probbly 4 or 5 weeks old theres another one doing the same thing but the younger ones are fine. the problem first started occuring about 3 or 4 days ago, what should i buy if it is a nitrogen deficiency.......also i have this stuff called super thrive ive givin them a little bit of this every now and again, but i havent used it for a while, did i give it to them too early or should i keep doing this

How often are you watering them? What type of potting soil did you use?

I'm a newbie myself, but if you used potting soil, you shouldn't NEED to give them nutes yet.. It almost looks like nute burn
Also at this stage I'd say you should only water them every 3-4 days

i water them a little bit every day, thanks for the advice on that. and i used jolly gardener? just something food city had when i remembered i need potting soil, what kinds are the best?. also i have them in plastic cups when will i need to put them in something bigger