You mentioned that it happened after you repotted. What was your new potting mix ? how did you care for them after re potting?
I see in the picture that they look purple. To my experience that could be the sign of cold environment or phosphorus deficiency.

You also mention that you feed them fish and seaweed fert. To my understanding, that is an organic thing and might not be readily available to the seedlings. Therefore your seedlings are suffering from nuts deficiency, mostly phosphorus.

On the off chance: Check underside of leaves for spider mites.

Gary

Same potting media I always use for transplanting my seedlings. This is their second transplanting actually. Same potting mix as the time before, so, unless they've suddenly developed a sensitivity. . . The soil they were in was still decently wet from the watering the day before, so after transplanting them I just moved them back under the lights. Like I said, this was their seconding repotting, this one much less traumatic then the first since they were already in their individual pots, I didn't have to do too big a number on their roots.

Yes, they always have a phosphorus deficiency, every doggone year it's the same song and dance and they're like that from the day their born. But this year maybe extra bad. Guess I'll plug in the space heater overnight and see if that helps.

The fish and seaweed is a liquid emulsion, so it should be useable (though, I admit it's new to me this year, so maybe I'm mistaken). I have been fairly happy with the results with the rest of my seedlings, from onions to peppers, the rest of the year with it though.

Spider mites eh? Man, and here the one thing I usually avoid from seed to spoil is insect pests. Hoping that's not it, but I'm off to check.

Looks like spider mite damage to me too.

i didnt see any mites, and its not something ive dealt with in the past. could they simply be too small to see? how would i control them if i cant see them? its like battling a ghost!

Zach, how many seedlings are we talking about?

I would say a good 10-13 seedlings. one variety (the other one whos tag i got mixed up) seems so far uneffected.

Spider mite damage. They are quite tiny. Tap the leaves above a white piece of paper. They are red in color.

Brought them in to the "plant doc" today and they did full imspection for mites and couldnt find any. they said from what i told them (same as i told yall) they said it was most likely fert burn. give them a good flush with plain water and Ill keep my eyes on. I will also check for mites again though just in case.

If it was insect or disease damage, it would be varied and patchy. If it is a soil problem, it would be consistent on all the seedlings. Best luck in solving this.