Yellow leaves curling down

Hey guys n00b here. For starters my setup for my baby is a 5 gallon top feed with hydroton pebbles with a layer of RW at the bottom and a 300w led apollo horticulture light set at 16 inches away. I've kept the ph in check. I'm using the liquid fox farms trio, only grow big and big bloom currently, and my baby is almost 3 weeks old(from seed) and her first set of real leaves plus the cotyledons are turning yellow and curling down. My new set of leaves are pretty and green. Big bloom got added a few days ago but after the yellowing started. I assumed nute def was my issue but it didn't stop. Have I not waited long enough? Or should I even worry since the new set looks fine. Pics included, first is of plant in question, second is my other baby whose first leaves have the slightest bit of yellow at the end. A week and a half younger

**edit
My cotyledons on the older plant are actually very green still, making this whole situation all the more confusing for me

 

I'm doing hydro, you're saying I should run it 24\7 from the get go?

 

Shouldn't*