HELP! Brown spots and yellow leaves..cant figure it out...

[jollygreenthumb]

Hi i have a bc northern lights bloomblow with a 430W HPS, DWC with 4 airstones
The nutes that i am using are flora nova bloom, liquid karma, sweet, and sm90. recently on one plant the lower leaves have started to get some brown spots and then turns yellow and dies. i have been researching for days now and have not been able to figure it out. i did a nute flush a few days ago but that did not seem to help. i noticed the roots were kind of dark but also read that flora nova nutes do that but i added a some hydrogen peroxide to the resovoir in case of lack of oxygen. i am almost 2 weeks into flowering and the problem is starting to show on some of the other plants. i have 4 double gum fems from white label seeds. the pH is about 6 and all the temps are in check. i dont have a ppm meter so idk what it is. im not sure if its nute lock out, salt buildup, or pH or a cal/mag def..please help! The leaves on the smaller plant start to curl up and die at the ends and also the leaves turn yellow with brown spots. friend says its overfert and cal/mag def. what do you think?

[DroMan23]

hmm to me it sounds like a manganese def, i jus went thru one , the leaves yellow a bit then get brown spots rite? thats what i had, and ye it might take a bit to spread to the other plants, i had 6 out of 9 plants infected, than i foliar fed wid some manganese, and they all cured than one got hit hard wghile the others were cured... im not saying thats ure situation but its what i jus went thru..

heres the link i was given it was real helpful to me has lots of pics hope it helps u out too....

http://images.google.ie/imgres?imgur...n%26safe%3Doff