UnIess the food is 100% organic, PH is the Single most important part of growing Period. In fact that is exactly what is the problem with these plants along with a bit of over watering.
[The purpling of the stems is the sure sign roots are damaged from soggy roots that won't absorb because they will not eat at the wrong PH.]
Pythium is a possibility in the root system and the end result is necrosis.
Pay close attention to nutrition the part about PH.
Lets them dry out a bit and with the next feeding make sure your PH is at 6.5 and in 72 hours you'll see improvement at the tops. This is only a slight speed bump if you want these plants to survive.