Yellowing

I have been growing tomatoes for a few years now, and I can’t figure what has gone wrong this year. Many of my seedlings, amongst a many different cultivars, are yellowing from the upper leaves down. Their growth is slowing.

They are in a heated shed, watered well, and fertilized with diluted fish emulsion about every 1-2 weeks.

Here are some things that may have gone awry:

2 nights of cold (7c), nonconsecutively, early on? An older bag of organic potting mix? Overwatering (though I let the top dry a bit before continuing).

More importantly, what would you do at this point?

(I am in southeast British Columbia, near the border)

Thank you!