For me that looks like a deficiency, maybe phosphorus?? But I am not sure about which deficiency. Maybe you fertilize with a bit of seaweed?

Thanks, Loewenzahn---I'll try that.

Kim, these don't look bad to me. The red seems mostly confined to the cotyledons, which are dying off (normal). Be sure the medium isn't too wet (it seems quite dense, but I'm not familiar with 'rapid rooter') and, yes, I would transplant them as you suggest, using a medium with some perlite in it, to get oxygen in the root zone. If you fertilize (fish or seaweed, good idea), do so with a very light hand.

Gary

The seedlings have taken a turn for the worse (healthy true leaves dying, and one seedling dead), but I've also learned some things:

Rapid Rooter are hydroponic plug growing media with a dense, spongy texture. I bought these without knowing a darn thing about hydroponics. While the plugs contain nice-sounding ingredients ("beneficial microbes" "composted materials"), they lack the fertilizer I've taken for granted in my seedling mixes.

So, I've transplanted all into seedling mix (the plugs have to be transplanted intact) and fertilized very lightly with fish emulsion.

Thanks, Gary and Loewenzahn.

I think eventually you'll save most of these. I would fill in the gaps with a very porous mixture and keep them damp but not waterlogged.

Good luck.
Gary