How are you oxygenating the roots? Your pics look like your using a tote with net pots in it... if you are just using an aquarium air pump then the odds are decent that the level of oxygenation is not satisfactory for optimal nute uptake. That could lead to all sorts of deficiencies. Also, you say that your temps are running 75-80. I assume that's the temp in your room, and without cooling, your res will eventually rise to that temp, which just so happens to be the level where you could experience root rot. It all kinda makes sense given you started noticing these probs as soon as the roots made it out of the soil and into the soup. Soil is more forgiving from both a pH and temp standpoint. I know a lot of folks on this forum don't have the luxury of copious funds, so acquiring new equipment becomes a problem, but given that pH and ppm are interdependent I would STRONGLY urge you to get a ppm meter. Otherwise you're just going to continue playing the guessing game.