Oh no - so sorry

Hope they can be revived

Maybe a photo would help? Maybe, maybe not. I assume you didn;t fertilize them yet. What type of lighting are you using?

Replace tray inserts. Then experiment by sowing again in them and also as a control take a plastic scrap tray, veg can etc. put 3 inches of mix in it and drop a couple of seeds in it. See if now both will sprout and thrive.

Doesn't make sense that the seedlings would die yet anything is possible.

George

Fertilizing at this early stage can burn the heck out of seedlings. But if you didn;t fertilize, maybe your light source is too hot. Using bleach really shouldn;t be a problem, if rinsed off a bit.

No, I have not fertilized the seedlings. I never do until about 20 days, then only some fish emulsion etc. I am not using liners. Could that be my problem? Again, I sterilized the speedling trays with bleach. I rinsed them off. Could some sort of pathogen be in the trays that the bleach didn't kill? I use just regular t12 fluorescents-one cool, one warm. They are not too close to the plants. Again, I have been using the same procedures for over 15 years. The coteledons are just drying up. Roots look fine. The rest of the plants are a bit droopy and not growing. 9 days old. I'll try and sent a picture.

Pick up a .99 tray insert cell 2 x 2 fill with your starter mix [Promix]. You can pick up a tray insert at a Nursery. Do the same with an empty can. Put in tomato seeds in tray and control seed in can. Now if you experience another die off, the variables are either water or mix.

George

You could sterilize the Promix and then the only variable would be water.

Sounds like you are doing all the normal things so I'm suspecting the mix. Any chance this was an old bale of BX that could have gotten contaminated? If a new bale did it look and smell normal when opened? No anaerobic smell like it may have been wet or stored wet?

Liner shouldn't make any difference.

Dave