Plants are 4 weeks old, in 4" pots. They germinated in these pots and haven't been transplanted.
Growing medium is Gardner and Bloome Seed Starter, containing peat moss, perlite, pumice, and washed sand.
Plants are under a pair of 4' cool white T5 fluorescent tubes, tops 2" below tubes, 14 hrs/day.
Air temps 68 day, low 60s night.
Plants get tepid tap water about once a week, lately with fertilizer added.

I started out following advice found here and elsewhere that tomato seedlings generally do well without fertilizer when grown in a typical seed starting mix, even one without added fertilizers. But when I noticed some of the first true leaves looked a little deformed, veins and undersides purple and starting to yellow between the veins, I took action. I applied a liquid fish/kelp combo at concentration of 1 Tbsp fish + 2 Tbsp kelp per gallon of water. As the plants grew the yellowing increased on the oldest leaves and I continued to fertilize about once a week, sometimes with the same organic formula, sometimes with Miracle Grow 1 tsp/gallon (the concentration suggested in Growing Vegetables West of the Cascades) hoping to cover all bases and hoping I wasn't making things worse.

Meanwhile, I scoured the intertubes to compare images and symptoms, and the closest matches were magnesium and/or potassium deficiency. Does that sound likely?

An example, a Sprite seedling.

Some of the oldest leaves.

Underside of same leaves. Actuallya bit brighter purple than in photo. Leaf areas most yellow seem to be most purple underneath.

Edges yellowing on 2nd set of leaves.

The newest growth looks healthy, so far. I've picked out a couple of sacrificial plants to experiment on, but I'm hoping someone here might have some advice in the meantime. Am I killing them with kindness? Or not enough? Any other causes to consider?

TIA
Mary Ann

It's cause is cooler temps than tomatoes prefer (something we can't really avoid), excess watering preventing the roots proper uptake of nutrients (let the plants dry out well before watering), and the fact that they are young.

Once transplanted to the garden and the warm weather stabilizes, it quickly disappears IF excess moisture levels are avoided and you don't get tons of rain.

So cut back on the water and cut yourself some slack with the worrying ;) and plant them in the garden as soon as it's possible. They will be fine.

Dave

Dang, I thought I was showing restraint with the watering. I've been letting the top of the soil get dry, usually every 7-8 days. Still too much? I could let it go a few more days and see what happens.

The fert I gave them is supposed to be the diluted strength already, according to what I'm reading. Well, I set my expectations low, and the plants are generally doing well, so I'm still ahead of the game.

MA