browning of quality marijuana

it starts off with my fan leafs turning brown around edges and then it takes over the entire fan leaf fairly quickly. this will happen to my flowering plants and veg plants. i have been doing fine for 3 years and all the sudden this has ben happening and i cant seem to correct the problem. Could it be lack of co2 or nitrogen im not sure but once it starts i cant seem to stop it. only 3 weeks left in flower and dont want to lose another cycle ,... anyone know whats going on????

i am not the best plant problem solver but lets get you started so you can get some answers.

you need to give alot more info.

grow room environment temp/ humidity/ph
growing medium
nutes
how often you water
lights
ventilation
pics

enironment is rather warm.. i use coco for medium with perilite and earth worm castings... water every four to six days depending on how hot it was.. 600 watt hps lights.. no ventilation and no pics.. but have growing this way for three years no problems

..

ive never used coco but it may be a PH problem,or nute burn.

do the leaves turn yellow then brown from lack of nitrogen or to they go from green to brown?

you may not get too many answers without pics.