The cotyledons are already fading which usually indicates over-watering. But you don't mention what they have been fed if anything. They very likely need to be fed as is always the case once they develop their first true leaves.

Dave

Along with what Dave mentions, what is the ambient temperature of the space? Unless it's a effect of light and photography, I think I see some purplish coloration. That can indicate lower than optimum temps.

I usually set my heat at 68, but the sun usually warms the space into the 70s during the day. The sill that it sits on does touch the window, an while it doesn't freeze, we did gave a cold spell in mid March and the radiant cooling may have brought soil temps into the 50s.

They need some fertilizer! They can get a bit chilly and that really stalls them which is why the ones that you planted later have overtaken them.....

Linda

yellowing usually only means two things.. time for fertilizer (nitrogen) or over watering. based on the use of the miracle starter i'll say it's the former. the mix is only good for the first 2-3 weeks of growth, they are past due for some fertilizer. i would fertilize them and transplant them a week later. the ones on the left seem to be almost due for transplanting into something bigger.. verify by letting the medium dry out a bit then gently popping one out the cube. if the cube starts to fall apart then there more room for growth, if it the root ball stays together than it's time to transplant.

Still no mention of what they have been fed, what nutrients they have received. The small amount of the slo release stuff in the MG isn't nearly sufficient.

Dave

Oh sorry guys. I haven't fed them anything. What should I feed them with?

I know a horticulture teacher who recommends Osmocote for seedlings.

"What should I feed them with?"

A frequent question here so you can do a search and read through some of them for specific suggestions. But the common recommendation is any diluted to 1/4 to 1/2 strength water soluble, well balanced fertilizer. Miracle Grow is only one of many that meets that definition. How much you dilute it all depends on the fertilizer you choose to use. But when growing anything from seed you must use something.

Dave