Whats the PH of your solution? How long have the roots been like this?

I try to keep the pH between 5.6-6.0, and it seems to hover right around 5.8-5.9. PPM is around 600 (EC 1.21).

I'm currently using R/O water (purchased from a grocery store dispenser), and the PPM of the water was 80 PPM.

The plants have only been going about 3 weeks total, so I think the problem may have started in the last week or two.

Lettuce generally does better between 6.0 and 7.0

Try raising the pH up a bit, I think you're a bit too acidic for lettuce. Tomatoes would thrive but I think that's probably what's burning your roots. I grow my lettuce at around 6.2 with good results.

I think H has got it right there, are you still having trouble at the higher pH?