Freaking the f out please help with thunderfuck seedlings(with pics)

"... i still recommend transplanting, 4" rockwool blocks are good. "
I will if I can figure out what is wrong with em.

I'm wondering if it is indeed nute burn.

Even 1/20th strength is not needed for small seedlings, and if you're just spraying the bottoms & sides of the cubes, the nutes will stay in the rockwool and continue causing damage.
**edit: I missed the sentence where you explained that you'd flushed the cubes, so this entire post may be of no use. Mea culpa.
One of my MTF plants has a similar light brown spot on a couple of places on one of the leaf margins, just like yours (except yours are maddeningly symmetrical where the light spots are), that appeared not long after the first diluted feeding (Jack's Classic "All Purpose" 20-20-20 @ 1/4-teaspoon per gallon which is anywhere from 1/4th to 1/8th the recommended dosage for a healthy adult plant). That tells me that I have fed them all they can handle, plus a little more, so it will be water only for it's next watering.

Not as big of a deal on a larger plant, but can be devastating for a tiny seedling, especially if the nutrients are still trapped into the rockwool.
Just a possibility?
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*Also, MTF has a habit of sometimes producing variegated leaves (streaks, patches, and even one whole side of a leaf starting out albino white), usually later on in the growth cycle, but can happen any time due to some aberrations in it's genetics.

I know it's not happening right now to your plants, but it might later on. The variegation usually corrects itself and doesn't affect growth.

Another thing I just noticed is how dark & red/purple the stalks are.
Mine at 4 days old and 8 days old:


Notice how the stalks had red/pink fading up to green by Day 4, and are almost completely green by Day 8.
It's almost like a Sulfur deficiency in that the stems turn purple and the leaves get chlorotic with some brown spotting in the leaf margins and eventually tip damage.
Just so weird that the tips are dying off like they are.
I'm stumped. But will keep searching as long as your babies are alive!

Hey Alex, Brother Doob asked if we could help you out with your problem.

Here's what I would do:

Get that dome back on until this problem is sorted out. Your plants are just on their first set of real leaves so still quite young.
Those rockwool cubes look real dry to me, you should have them moist and then have a bit of moisture in the bottom of your dome as well so the roots start looking for water.
Calcium deficiencies classically produce distorted leaf tips and then dying leaf tips (which is what your little yins look like they are doing) I use potting soil and live in a country with heavy calcium in the water so tend not to have this issue. The only thing you have changed is going from RO to Tap water so this has to be your problem (I doubt removing the humidity dome would be as seedlings can handle environments pretty well - the medium they sit in is a different story though)
Check your local tap waters calcium content and also look into it's ph value as well. Monitor the RH in the dome and perhaps give the clones some fresh air by removing the dome a couple of times a day. Go to your local DIY store and buy some Distilled water if you are not sure. Or purchase some Rhizotonic which will get those little babies back to life in no time at all. Rhizo has an NPK of 0.6-0.2-0.6 i think...
Can you see any of the roots coming out of the rockwool yet?

too much light, forget nutes, get them farther away from the light, or put them under a smaller wattage light.