Leaves browning on one of my plants - help? (pics)

"in a closet under 400w HPS and 65w CFL "

Heat maybe?

 

Maybe...IDK man, I've had heat problems and some of the ends of my leaves are yellow/brown on the ends like that.

But they didn't/dont have the yellow spots in the middle of the leaves like some of you pics show...

 

So you have about three to four weeks to go. Fan leaves yellowing out and dieing off in late flower is common, particularly with sativa strains.

That's low. Not outrageously low, but low. The happy zone is 6.5 to 6.8. You are in magnesium lockout territory right now. pH is always the first thing to check, and when it is off, you should fix it.

I would flush it with pH 7.0 water. Take pH readings of the runoff after the 1st gallon, then again after the second gallon. I bet the second reading is appreciably lower than the first reading. And the third reading (after pouring the third gallon) may be lower than the second. It can take some water running through before the salt deposits start to loosen up. After that you will be checking after each gallon poured, waiting for the runoff reading to turn upward. It will, eventually.

If you decide to flush, you might as well aim for 6.8. If you aim for 6.5 you may find yourself flushing again in a week.

Yes. Your bottled nutes should cover your plants nutritional requirements. When they fail to do so, it is usually because of low soil pH or an overabundance of one nute locking out another. For instance, potassium can get locked out by sodium or calcium. But I don't know of anything that specifically locks out phosphorous. It is normal to get some signs of phosphorous deficiency during flower, but it should not be extensive. So say the stickies.