Bottom Left and Bottom Right you can move to individual small pots. The overly variegated one - you can try watering with a fish emulsion, weak solution of Epsom salt and prayers. There is a possibility it will green up.

The upper row is still work in progress.

Would Alaska Fish Fertilizer 5-1-1 be ok to use? It is available locally for a reasonable price. I have never used fish emulsion and don't want a huge quantity but would hate to lose my baby leaves! Thank you, Irina, for your advice.

Yes - this is the one - it is sometimes sold in small bottles. Make sure that you use half less than the does on the bottom.

There is another good use of it. If you decide to leach the soil - let say your plants are not repotted fr more than 4-6 months - amd you do not plan to do it immediately - you can ran a quart or 2 of a lukewarm water through the soil in the sink - normally just tap water works - but if you want a boost - put a several drops of a fish emusion in this water.

I wonder if the fact that you have them growing in a muffin tin might have an effect? (Whether from the aluminum itself or chemical compounds resulting from interactions betwixt the aluminum and the media and/or water ....)

I think the detriment is not so much in the material - but the shape and size of the containers. The soil looks like Promix to me - and it is finely milled - so it gets settled when watered and limits the air supply to the roots. Smaller container - solo cup or 1 oz cup with a good size opening on the bottom - lets more air and better breathing to the roots - and as a result - faster development.