Browning/ Dying leaves, deficiency? 8 weeks flowering with pics

Ok i've got a worrying problem with one of my plants, from the bottom up it has started to yellow, which i hear is quite normal at this point in the grow. However, im having problems with the middle of the plant now and its spreading upwards to the bud leaves. I can only describe it as the leaf edges starting to brown, with a purplish tinge. This always starts from the tip of the leaf before affecting the edges then moving to the cener of the leaf, once this happens the leaf wilts and dies. My WW is in the 8th week of flower now and only a few of the pistils have changed so i figure ive got 2 weeks left or so. I can only think this is happening to this plant more than the other because it is in a smaller pot- i dunno

Anyway its in biobizz allmix, fed biobizz nutes - bio grow -1ml biobloom-5 mls and molasses 5 mls. I have given it a dose of epsom salts too as i thought it was mag defficiency but im not sure- anyone know????

What temperature is the room you are storing your plant in? Also, what is the temperature of your soil. Purple color like that typically means a phosphorous lock up, which occurs at about 50 degrees f. We are entering a very cold season where I am living and recently had a very similar problem.

my light time i have operating during the night to help combat the low temps where i live, when the lights are on the temp is 65ish humidity 40. Whats strange is this is only happening to one plant, which is in the smaller pot, so phosphorous def? shall i give it some bone meal then?