root rot situation update

so yeah, all my plants survived the root rot and grew new roots over the brown old ones. The first pic is of the plant that got the worst of it on the stem. Its also the same plant on the second picture. I took it because I dont know what the browning parts of the leaves is....Iron deficiency? Its the only plant with it and I know its not heat burning because its not crisp, and its the plant furthist from the lights.

Any thoughts?

Also, I bought a ppm meter and the best I can do for the fourth week of growth is about 1,000 ppm. My water is about 400ppm and I cannot seem to find an INexpensive water filter....???