What I’m doing wrong?

The moment I put the lettuce seedlings in the recirculating system this happens to the roots. It happened in about 4 to 6 hours.

The first time I thought it was pythium and I reset the complete system and cleaned it with bleached water. Is root rot that aggressive to occur in less than 48 h since a reset?

Cucumber and tomato seedlings roots are fine. I'm starting to think I'm shocking this specific plant when I move it to the system.

Stats:
RO water
Masterblend tomato @1.2 EC
pH still raising but actively kept in 5.5 – 6.5 with nitric acid

In case is pythium I'm gently adding H2O2 and Bleach alternating them, in half of the usual recommended concentration.