Ok....so I've gone back and read through this thread(bored until the eagles play/fu giants!). Now I saw through your pics that they were germed in ffof and had NO issues then transplanted into 3g with more ffof and no issue for TEN days after that. So this DEADLY soil didn't do any damage when they were seedlings and MOST susceptible to nute burn. This tells me something was added or your ph IS OFF, due to it taking about two-three weeks to show in a ph balanced soil like ffof(6.3-6.9)
1......you say you havent added ANYTHING other than water...... correct?
2......some advised you to add cal/mag, you never responded....did you? A calcium toxicity will lockout ALL micronutrients and MAG which your latest pics ^^show, A magnesium toxicity will give you a salt build up which will kill everything!
3.......your meter's probe may be broken/malfunctioning
4......you are overwatering(sorry but true IMO) and with cold temps at lights off or on(with leds) will cause multiple problems with your root health, and kill your plants.....FYI..I watered up to eight days in between with my leds in temps above 90f and in a COCCO medium!! it takes a long time for water to transpire under leds/ it's the #1 killer that I've seen for new led users...........
good luck and I'm not trying to be a asshole towards you/ just helping