Basil (rusty leaves)

Ok peeps. I have some basil in my west facing windowsill in flood/drain hydroton. Floods 5 mins every 4 hours. EC 1.4, pH 5.8. Canna Aqua veg with added Mg. Transplanting seedlings from soil at 6-8 leaves.
The leaves keep going 'rusty' and the plants are dieing. Everything else in there is ok (mint,peppers,lettuce seedlings). Night temps drop to 55F and days get up to about 80f.
Any ideas?

Nute burn?

could be that they don't like it being 5.8 PH. My basil are in 7.0 PH DWC system and they aren't having the rusting problem. Mine also only deals with 70F temp range. As for the feeding I do I only give my plants 0.5ml per 1L of water even though it says to give them up to 3ml.

Thanks guys. Have raised pH and dropped EC (basically watered down my res with my 7.8 tap water) and they have picked up a bit. Also noticed those closest to the window are doing better so added reflector (correx sheet from a long-abandoned nft experiment). It seems to be a balancing act with different plant using the same res.