stunted growth/ yellow leaves...HELP ME PLS!!!

i am a first time grower, i have a 4 pod hydrosystem from esoterics hydroponics, im growing 8 plants, 3 george's diamonds, 2 orange bud, 2 mazar & 1 blueberry.
they are in the 6th week of flowering 12/12. however it appears to me flowering is very slow. i cant say there's buds yet- but it is pretty much covered with hairs. im growing in a grow tent with an extraction fan, 400w lamp.
My problem is alot of the fan leaves have completly turned yellow as if they are too close to the light and are being burnt(they are atleast 40cm away). so it is not that. the leaves yellow from the outside towards the centre vein.
to be honest my cf level got out of hand, it and was at +30 for a few days. this was cos i was putiing ph adjusted water with nutrients into 50lt res. and did judge the salt build up in my tank. i was advised to flush tank and ran ph adjusted water only for 2 days. thats pretty much when this yellow-burningness started. i keep ph at 6 (recommended for clay peble medium growers) now cf level is 17 and drifts up to 22.
i empty tank every 10 days and add fresh h2o.
what could help?
i am using a quite alot of ph down. my tap water is 6.8- 7.0. could this effect?i bring it down to 6.0
what about reverse osmosis? in veg stage plants showed signs of low Mg and was recomended rev. osm. filter but seemed to correct its self when changed nutrient to bloom-richer in Mg. so i didnt get one (expensive)
i've also heard that it could be too much phosphorus, if so how could this be corrected?
the seeds are supposed to be ready to harvest in 3-4 weeks...
Where is my 350-400gr's per 1m of plant????:(
will this all bloom suddenly at the end like tomatoes and im worrying for nothing????
if someone has gone to the trouble of reading this, thanks alot for your time....:rasta:

are you using a bloom formula? How much nutes are you feeding the plants? What are there current ppm levels?

If all the strains are doing the same thing then I doubt its a strain issue. I hear that sometimes different strains can turn yellowish towards the end. Sounds like not enough of the right level of nutes to me. You will need to feed them more as they begin to produce fruit. I think 6 weeks is about 2/3's through the process. Figure 9-12 weeks for a proper flower is about right. They will pack on more and more crystals in the final weeks.