whats the problem? Should I trash the weak?

One of my plants has yellow "marks" on its first set of "true" leaves, some on the second, and nothing on the newest ones yet. A second one has really large leaves and is dropping heavily, maybe from overwatering but Im not too sure maybe something else is wrong, sorry I got not pics but could this be genetic or a nutrient deficiency? the seedlings are less the 2 weeks old and I have several which look very healthy should I trash these? I know this isnt the bug forum but I have noticed 4 spider mites in 2 days, killed them and inspect the plants with a magnifying glass 2-3 times since the first sighting, I have not seen any new bugs and I removed everything from my grow room and sanitized the area.