Brown in the middle of the leaf, new growth only. Never seen before! DWC

Hey guys just wondering if any has seen this before?its a DWC.

This plant had root rot early with 2 others ( I lost the other 2).

I moved the tent to a cooler location..... this one 1 plant I was able to save,although lost most of the roots. I treated and put back in soil, let the roots grow again, bleached everything and transferred this one back. Roots have just taken off. And in 4 days is indescribable. The new growth on top is slow which I'm assuming ita tryna grow it root base back. The new growth right in the middle is brown only in the middle. Very weird. Does this look like a sign of nute burn?

I dont wanna change the water yet as last time when I changed the water that's when it hit, from what I read it best to let it build up bio film the first month so the roots are stronger.

The ppm was 330 yesterday and today is was 333 with a small amount of water gone. So it didn't change much. Should I lower the ppm is just leave it and let it do it thing since the root are growing well?

Thanks for all the help, i just couldn't find any info or pics to match what I have.
Thanks again

Rh is 50 to 55
Water temp is 68.5f
Ppm is 330 ( my water is 120) then the plant was still looking a little like faded green so I bumped up the ppm from 250 to 330.

Current culture veg a and b
Uc roots
Ph down
 

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I get these brownish red spots on new seedlings(newest growth) once in awhile if theyre over watered. (Soil) itll grow out of it usually. Not sure about dwc
Maybe it could've just been growth from when it had root rot, now it's just growing out. Well find out and see I guess. I just dont wanna mess with anything atm since things are working. Thanks so much for that, makes me feel better lol
 
Have you seen any bugs flying around? I put foil on top of my hydroton one grow, very similar to what you've done and the fungas gnats loved it in there! It also kept everything very wet, rotting wet. Keep an eye on it.
 
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