I need advice on plant problems

About week eight in my 10 week grow cycle my leaf edges started to turn brown, now it's week nine and they are all doing it and they are curling, my ph is kind of low, between 5.0 & 5.5, is that it or something else? I grow in a cabinet and use FoxFarm soil and nutrients with a 400w full spectrum hps light source. Any help would be appreciated.

ur ph is way to low for soil u want it between 6.3-6.8, and it looks like u have a spider mite infestation can see webs in the pics thats ur problem cut that bitch, and take better care of your plants, on your next grow clean out ur grow area with bleach u got spidermites bad