Purple stems and dying leaves.

I gotta little problem here. Ive got a 2 month old flowering right now and it seems to have taken a turn for the worse. All the leaf stems have turned purple and the leaves are dying. They turn yellow from the tips inward, then they get brown and crispy until they fall off.

I have 4 more larger plants of a different strain in bigger pots that are doing the same thing. I'm thinking it is a nutrient deficancy of some sort but I'm not sure what nutrients they need. I have some general hydroponics 3 part nutrients but I've been holding off on using them because I'm not sure how much to use.

All plants are in foxfarm ocean forest soil and they are under a 250w HPS light.

Here's a couple pics of the smallest plant and one of one of the larger plants. Any info will be much appreciated.
Thanks guys!

whats your ph at?

What is your pH at and also why are they in such small pots? Additionally how often do you water and what nutes do you use?

Im not sure about the soil PH but the water ph is around 6.5. I gave them a vry small about of nutes about a week or two ago(1/2tsp of each of the 3 parts per gallon of water). I have general hydroponics 3 part nutrients

I water the plant in the party cup(I wanted to try a party cup grow) every day almost and the other every 2 to 3 days(they are in lager containers).