*Apparently a few stray seeds from food prep / slicing & dicing tomatoes or tomatoes that go bad get in the compost.
That said, thanks, I may pick up a bag of the Hapi-Gro Timberline 40-lb Organic Compost and Manure (1.88) and / or Peat Humus40-lb $2.28) that you recommend. We are low on such material. The lighting for the seedlings is mostly artificial (grow bulbs - no good unshaded windows to grow stuff) but I've also tried starting the plants outside - granted that was in early summer after the indoor attempts failed, and those may have received too much heat and light early on. (We are in the Mid-South, Zone 7, and the seedlings would have been about 5- 10 days old right at summer solstice, both times, IIRC.) So that might have been a different problem.
Water, well (pun) it's good tasting well water, a bit "hard" but not bad, I drink a lot of it untreated, and I haven't toppled over yet. I guess I could test ITS ph or just switch to collected rainwater. (There is plenty of that available in the spring, here!)
I'm guessing maybe when watering for being away 2-4 days the plants are TOO wet too long. Or maybe it is the light, or, some sort of pathogen* (despite the new soil) that plants 1-2 weeks old in small reused containers are vulnerable to. It is NOT too much plant for the container -- store purchased plants are probably 20-100 times more plant mass for the container than the seedlings, at the point the seedlings keel over.
*I also suppose it is just barely possible there is enough juglone in the well water to damage seedlings at their most vulnerable(?) stage(?) -- we have tons of hickory trees in the area (but not near the garden.) We use the same water for the garden though - no apparent problem, but then that's on older plants. This time of year or a little later, there's no need to water the volunteers - they get plenty of rain.
Maybe the grow light needs to be closer? Within a foot or less? (In past years it was a 40w. fluorescent bulb about 30" away. For this winter I replaced it with a 5000 lumen 5000K LED fixture - it worked ok for plants (including a couple mature tomato plants) I overwintered - see below).
I have NOT yet had a chance to start from seed tomatoes this year -- figured I'd get on here and ask some questions 1st, actually, since the past attempts were a waste of time / disaster.
Also, that light is not on a timer, so maybe the "never off" light messes up young plants even more than people? (It's never hurt other wintered plants we have, so far as I know.)
The problem when I tried growing seedlings from seed "outside" in small containers may be a different issue, as noted.
The fan is a good idea, but how to not forget the dang thing when I'm away, and dry out the plants even faster? It'd only take one mistake... Maybe run it on a separate timer so it's only on an hour a day?
Sounds like you are far from a novice! I would recommend getting some 20oz paper cups (or 60 18oz for 12.00) and putting a couple holes in the bottom and using them to plant in, with good soil. On watering, I only water our many (thank God) growing plants about twice a week at the most.
Sometimes if you reuse containers your seedlings get cooties.
I forget what the ratio is for bleach to water for washing containers.
Some people bake their soil to sterilize it.