Need a little help. Her leaves are turning yellow.

This is my first indoor grow. I can rock it outside but I'm struggling inside. I've been feeding Neptune's Harvest fish at the recommended dose. I've started using a little seabird guano as well for the phosphorus. Organic soil mix of pearlite, store bag soil, and coir.
The light is a "1000 watt" Chinese led board light off Amazon. She was doing great in veg and now this.

Is that “store soil” fortified? What percentage of this mix is coco? Watering by weight or some kind of schedule? Container size and volume of liquid used when you water/feed?

I discovered the soil was getting too dry between waterings. She is doing well now.
Thanks for the reply hotrodharley..