slight problem.. leaves light green yellowing slowly from the tip

This plant is white rhino 5 weeks into flower grown under an 600 watt hps .. phed tap water.. temps around 77 humidity around 40 percent,grown in happy frog soil(with a thin layer of guano and castings on top) I water every 2 to 3 days 1st regular water.. then foxfarm tigerbloom, then molasses water. Some of the tips of the leaves that are on the buds are turning yellow curling up slightly.. plant just looks light green alot more than about 2 weeks ago .. alot of lower /middle leaves are very yellow im just not sure exactly what it is really.. I tried to diagnose it with the guides on here but id really rather get some feedback.
http://i1331.photobucket.com/albums/w599/jambi12621/20130811_112443_zps77853077.jpg

N-P-K. Looks like the p, potassium. See how the edges start first? look up potassium deficiency pictures. in flower cannabis need higher lvls of p.

calmag def

im willing to bet it started around the 2nd week into flowering?

Super sorry for the late reply guys! it started around the 4th week of flowering, its getting a little worse.

so you think its cal-mag, iv read into that but idk, it seemed to come on quick.. so i should just try some cal-mag stuff? or is there anthing else i can use

I've read alot of your old posts about the problems with promix. & Well I had the exact same issues with FFof. Like anywhere from 2-5 weeks of flower plants get deficient. But through veg ALL my plants have flourished in FFof with NO veg nutes. But come flower time a few weeks in the problems arise. Anyways I was wondering did adding lime fix your problem or should I just water with calmag throughout flower?? Thanks in advance homie. I got a bottle of General Organics calmag. And I water with RO water..