Help with my first grow - Yellow Spots and dying leaves!

Hello, I am a complete noob at growing.. I am growing indoors with a 250watt hps light in BioBiz All-Mix soil. I water when the pots feel light and its dry when I stick my finger all the way down. The cycle of light is 18hours a day right now for vegitation. The temparature averages at about 78-80 degress fahrenheit, and I have a desk fan on constantly. They also enjoy the music of Jimmy Hendrix, Bach, John Coltrane and Miles Davis. I tested the PH of the runoff water and its around 6. I recently nuted the two biggest plants a couple of weaks ago, but where showing yellow spots before I nuted. Please tell me what I am doing wrong!!

The two smaller plants are Iced Grapefruit feminized seeds and the big plant is cheese and the bad looking skinny plant I think is haze. The cheese and haze are both bag seed.

Look to be suffering from lockout and maybe overferting.
Not sure but I think 6 ph is a bit low for a soil grow.
I'd flush the pots with 1/2 strength nutes at ph 7, leave them a few days and maybe give them just water when they need it for a week or so.

Hmm, I was really thinking nute burn until you said they were showing yellow spots before you started nutes. I'd maybe give them a good flush anyways for a couple watering cycles and see if that helps.

I have same idea with witelightnin .....
Only use flush (fresh water with correct ph {6.2-6.8] 3 times )