Quick yellowing in advanced flowering. Plz help

Hi all, tanks for having me, first grow, first post ;)

Before we get into business i would like to anticipate tat i ave done my thorough researces about the issue at hand and found many similar problems, similar, but not same, and would like some reassurance.

For starters the system:
1 400 W HPS with full ventilation
15 l pots (that's like 4 gallons to you yankees :))
All mix soil (bought from growing store)

Temp and hum: 76.8 Average, 40% Stable

Nutes and feeding:
Bio**zz complete cycle as suggested by producer =>
BioBloom: 6ml/1.5l
TopMax: 6ml/ 1.5l
Biogrow: 2ml/ 1.5l

The amount is dosed on 1.5l (about 0.4 for you guys) because that's te actual amount of water i use, a 1.5l water bottle every two days, and fert @ every watering (as suggested by above mentioned producer). Had a great veg fase, when turned to 12/12 problems started piling up.

Actual period of growth: 4th week just began

PH: ard to tell, i guess about 7 from the color in the probe

Seed: Diesel feminized (i tried looking for posting rules but found none so if perhaps i'm infringing some rules i apologize)

So the problem is last night i gave one of the two ladies (i have alternate cycles for watering my 2 plants, so it keeps the umidity more stable) it's usual dosage of water and nutes, today as sun rose (for them) i noticed that the watered plant has become very pale all the way up to about 1/2 height, bottom leaves and young ones had already been yellowing for some time, but they got absolutely no light and i reckoned it was pretty much normal. Now all the plant is pale half te way up (it has about 14 internodes now), except on stems!! stems and their leaves are perfectly fine, excpet maybe te laves quite curled in. I noticed there was a beginning of mg deficiency a week ago or so, from an apparent subtle yellowing of the blades of the bigger leaves. Didn't take care of it coz i didn't want to ad too many minerals to te soil. Now bottom leaves are plain yellow and the plant goes fading up to the big leaves in the middle which are a little curled up and have a yellowish line all over the main veins. I've looked through some charts and perhaps most of the problems i see can be traced to a Mb deficiency, is this possible? I ask this because i must confess that i have a very mild fungus gnat infestation that's been going on for a few weeks, i kept them under control wit garlic-tobacco spray and by covering wit sand and replacing top inches of soil every now and then. Still when i water some come out and disappear soon after, as i've cecked they're unextinguishable, but also not a treat though more of a nuisance, aamof what they do is eat organic material in the soil => thus nutrients=> thus perhaps nutrient deficiency?
Must note: I have two plants, which had the same treatment all the way long but only one is showing these problems. One is short, bushy, green, sturdy with only a slight browning of the blades of very bottom fanel leaves and some general withering down at the very bottom, so i think she's doing pretty fine, the other one grew all long (10in taller than the other) skinnier in foliage and since i grew it without supports it grew sligtly crooked, which is fine coz it occupies all the space available, gets more light and evens out with the other plant, so i really can't figure out what's going on here...
Suggestions? Comments?

I apologize for making your eyes hurt,

Cheers!

Really? No suggestions or comments at all?

You state your pH is 7? If that is the case, it is too high and causing nute lock. You need to be more precise with your pH readings as that is the key for your plant absorbing nutrients. You need to keep your range between 6.3 and 6.8, with optimal being 6.5

I have included a chart so you can see what is being locked out if indeed your pH is 7

GTD

Many times yellowing late in flowering is actually normal. This is just an annual plant, lives less than one year and dies, turning yellow before it goes. How's the bud formation? They might just be finishing in normal fashion.

Get some Pyrethrum, and kill the bugs. It's made from chrysanthemums, and should be fine. (low toxicity) Over a short period of time, the chemical compounds in the stuff dissolves. The more you dick with 'em, the more damage you're apt to do.

Pictures would help, but you'll need a few more posts under your belt before the web site will allow it.