I agree that the young seedling with the yellowing cotyledons is having a problem with water. Very small plants need very little water and nutrients. Even large bushy plants only get watered every 3-5 days depending on drainage. Your seedling seems smaller than what it should be. The roots might not be getting enough air due to excess water in the dirt. With seedlings it can be a balancing act with water. Usually when you water, you want your dirt to be thoroughly wet, then you want it to get thoroughly dry. The bigger leaves will droop when needing water, and will curl under after watering or if there is plenty of water in the soil. The rub is not letting the seedling itself dry out. I doubt your young plant is using much nutes. I hear DP can be a difficult plant to grow, so if you loose it, I wouldn't take it personally. Also, you can't listen and believe everything you hear. You're going to hear ten different answers when posting deficiency pictures. Use proper pH. I'm pretty sure I've lost a few seedlings because I assumed my pH was good. It changed from year to year out of my tap. Fix your water issues and pH before throwing chemcals at it. When nutes do get added, make sure to do it gradually. You might consider some dolemite lime when transplanting to help regulate pH(keeps it from droping too low). I lower my water pH using white vinegar but there are other choices out there like lemon juice. PH isn't as important in soil as in other growing methods, but you still need to keep an eye on it. A cheap way to check is using hydrion/litmus papers. Not the easiest most accurate way to tell, but its cheap. Sorry if I got preachy. Good luck, and do lots of research!