My plants had the same issues.
I found it was two main things.
1. Over watering with nutes.
2. Light to close to my plants.
I fixed the problem fairly quickly buy watering with only water and not as often for about a week.
Moved the hps lighting a little higher.
Removed all the really curred leaves.
Now my plants are getting back to norma, are much better and stronger.
Hope that this helps you, it worked for me so it may just be the same
issues as you have.
Happy Growing,
_forbidden_