Hi guys almost 4 weeks to go..what I should do ? D...

you cant really go under 16, with most strains 12 is way too cold. you must fix that

First off 12 is way too low for indoors, is there any pest in your crop? how much ppm in your solution? How wet is the medium?

Also, in winter, setting your exhaust at the base of your tent instead of the top, will make the hot air stay longer in your tent and will also help warming up your soil.

what I see here is a combination of thing. Pic 1 and 4 clearly shows P deficiency. Pic 3 shows some colour that could results from low temps. Because speaking of low temps, 12 is too low, and at this temps, plant have issues up taking nutrients and the first one that become less available would be phosphorous and magnesium. To fight low temps you should raise your plants: anything can be used like milk crate, rigid insulation board, wood pellets.. You could switch to Hps light, set a small heater in the room the tent is, maybe?? Also when you water, take the time to warm up the water at the temps of your body before giving it, so it will help keep the soil warmer. I am having the same issues. I've set a veg tent, lite with HID, that exhaust at the floor of my flowering tent. It really keeps soil warm. Also having a lot of ventilation, combined with the use of fabric containers, really speed up the watering cycle and plants loves it!

You look like you have more than four weeks that are left. Just water like normal with only a bloom supplement. The nitrogen is only needed a tiny bit at flower. the yellowing is the plants way of telling you there close to harvest and the nitrogen isn’t taken in. The p and is are the big needs at the rest of the grow. The yellow and reds on them look great . And patience will reward you. Good luck grower