plant necrosis??? whats wrong?

He guys I am posting because I do not know what is up with my plants. It looks like I have a necrosis problem. I have tried to contact Humbolt nutrients since that is the line of nutes I am using unfortunately they have not been of much help. I have been following there online hydro program which is on the pdf attached


I am on the week 4 program currently I have made some substitutions in the formula which I was advised by my grow shop was ok since my substitutions were almost identical,with the exception of sea mag n sea cal since they are completely separate in humbolt line.
In my aquamist set up I have 2 strains 1)is the ak47 and the 2)is blue cheese
The blue cheese is the one showing these signs
I am not using humbolt roots.
as substitute for:
white widow I am using bushdoctor microbe brew
Flavorful I am using Floralicious plus (by general hydro)
And last Sea cal & Sea mag are replaced by cal-mag plus (by botnicare)
Now my rez ph is 5.5
Rez temp- 68-73
And my water is filtered with a declorinating filter with an ending ppm of 50
I have included pics of my leaves for the most part it is on the secong set of leaves from the tops of plant and on some occasions on 1st set , some of the bottom leaves are showing as well.

Help is much appreciated at this point

 

i was told by number of people that, that is the ideal ph. tds of solution is 660. and temps will range betweek 65-74.
under a 400 mh currently about 6-8 in from tops with good airflow