Weather is terrible here, like early March instead of late April, but they have good indoor lighting placed very close to the plants.

They are planted in Happy Frog potting mix, which tends to drain out it's nutrients in a couple weeks but this has seemed more than the average issues I've had.

I fertilized them only with some Dr Earth liquid 3-2-2 diluted down and when I transplanted them a couple weeks ago.

Last Wednesday I gave them a sparing application of a dilution of organic liquid stuff called Grow (from company called Age-Old) which was 12-6-6 which didn't seem to have much effect.

I did two soil samples with the Purple capsuples, both showed an midrange purple indicating they had adequate Nitrogen. This test kit is at least 5 years old, but I tested two samples, one from very blanched sad looking tomatoes and the soil from some very healthy dark greens plants and both measured midrange.

If you still need the answer, it looks to be iron deficiency; the "new" leaves show first, as opposed to nitrogen deficiency showing in older leaves.

Also, the very pronounced mid-vein is a dead giveaway with the yellow leaf margins

Good to know thanks!

They did come around and looked fantastic about 2 weeks later with a richer feeding mixture (but still lighter than full strength) and letting them dry out more.

I really struggled with soggy potting mix this year, I think I was so viginlant and involved that I watered too much and too often. In a typical year I would get distracted and nearly kill them off from forgetting to water. Underwatering is key. Possibly there is a relationship between soggy mix and inability ot uptake iron and other nutrients.

Yes, in a heavy clay soil. the "system" becomes anaerobic, and many nutrients do not breakdown like they're supposed to, or they become inaccessible due to the reduced oxygen by binding in a different way. but in your case, overwatering WITH the reduced concentrations of fertilizer leeched all the additives you added. the water "flushes," or leeches the fertilizer (salts) from the soil with each watering due to the potting mix (it's loose and light, and readily flows water).

During this I'd wondered what the most delicate stages of the Seedling where and what strngth of fertilizer would be too much. Typically I transplant into HappyFrog, things look good for a while and then they often go pale in 2 weeks. At that point the seedlings are probably 4-5 weeks old and can probaby take 1/2 strength fertilizer?

Does the Happy Frog contain Coir? That can hold too much moisture and plants don't do so well :(.

At 4 to 5 weeks, there should be a second and/or third set of "adult" leaves, you should be able to start using 1/2 strength liquid fertilizer, the in 7 days, 3/4 strength, then in another 7 days, full strength liquid fert. keep watering in between, and dont rush the results.

I think it contains coir.

I think I just watered too often was my main issue