Mg deficiency?

My leaves have slowly progressed from simple yellowing, into this. Leaf tips are curling up and twisting on older fan leaves and the problem is working it's way up the plant. There is no green in the leaf veins, just yellowing. The tips are getting crispy (dead).

Indoor
Soil
Fox Farms Ocean Forest
Dolomite Lime added
Hmmmm....pH'd tap water at 6.6 run's off at 7.1, too much lime? Why would too much lime be presenting a problem now?
Tap
7.something water source
Day 51 flowering, 89 days old total.
Fox Farms ferts.
Stopped heavy feeding weeks ago, currently flushing. Fully flushed plants 3-4 days ago. (5 gallons ph'd water through each plant, no ferts. added.)
600w HPS, 15-18 inches away.
75-80 day, 60-65 night.
40-60% humidity
12/12
Passive intake ventilation

..Or a Potassium deficiency?

it's the flushing that has killed your plant. why flush the food out?

Your runoff is 7.1? Thats too high.....you're locking stuff out. Flush with water at a lower ph (say 6.3) until the runoff is NO higher than 6.7. Then give nutes at a low-but-in-range ph level.

If you've been "flushing" or not giving nutes for several weeks they're probably starving anyway. But correct your ph first.

Severe N deficiency. pH your water down to 6.2, also check your meter calibration. Feed with a grow fert at 1/4 strength ONCE to slow the yellowing and then return to bloom fert to limp through to harvest.
Read the flushing why and how to do it sticky. Very important.