Seedling yellowing

don't use phosphoric acid for ph down use nitric acid for ph down its already burn they don't grow happy growing .

Coco is not a forgiving medium to grow in, especially for someone just starting out. Coco will bond with magnesium and typically you will need to rinse the coco with a solution of CalMag to help with this before you grow in it. Coco also has no nutrition for your plant, so you will have to provide it with all of its nutrients. Typically a water every day or every other day is required and that watering should have roughly 10-20% draining out the bottom of your pot. Your plant does look like it is experiencing some issues with pH. I would mix up a batch of fresh water at a pH of 6.2 and add CalMag into that. Add enough to your pot where nearly half comes out the bottom. I hope this helps.

In coco it can be your coco ph is off, or you are giving it the wrong feed amounts. Your run off ph should be 5.8. Your feed tds should be around 400. Calmag is also needed in coco grows with them being high irrigation grows. Limited information makes it hard to pin point which issue it could be. But you should have a idea.