Hope/Help for these sick seedlings?

[GaryStPaul]

Hi, folks. 90 seedlings, sown 21 March, grown under lights in basement, transplanted into cups on 15 April, moved to greenhouse 28 April. I followed same procedure I’ve used for decades (temps in the mid 60s, good light, gentle fan, no damping off), but I’ve never seen such weak, sickly plants, now about 6 weeks old. I’d hoped to plant them out three weeks or so from now.

All the yellowing suggests overwatering. The only difference this year was that I used a potting mix that—I just discovered—contains “Moisture Pro” water holding crystals which I hadn’t noticed. (It also contains some nutrients, 0.18/ 0-.06/ 0.12). In the past I’ve used Pro-Mix without problems. If all this water retention is the problem, I’m wondering if I can get these seedlings back to health by repotting the entire lot, into, say Pro-Mix. I would very much appreciate any help with the diagnosis and also the Rx, if any. At this point, they look like they’re on their way out. Pics follow. Many thanks, Gary.