It's hard to say now because you've used epsom salt but it appears that you have nutrient lockout of some sort.

1. Either your pot is too small and you're rootbound really badly.
2. Your soil ph may be 6.5 and water may be 5.0 but what is your soil runoff ph. That's what we really need to know. The reason why I say this is because it looked like you either had a ph problem or a deficiency.
3. How much did RO did you flush with? Did you add calcium or magnesium? This is important especially after doing a flush.

I'm going with either PH or rootbound on this one, but it's hard to say on this one because you may also be suffering lockout due to the epsom salt.