Why are all of my coco grows dying early?

@Organoman and @ezzjaybruh are right. to much light and to much salt IMO.

I cant see any powdery mildew anywhere here my mate dont listen to anyone,powdery mildew is like a spreadout moth balls,your resin looks ok to me no mildew on your resin,look thru a jewlers loop,i would say you are harvesting too late-your growing northern lights auto? Your cutting way too late,dont keep them 10 or 12 wks once youve switched cut after 8 wks exactly now theyre established cuts ull see the difference,or go for genetice that finish 65-75 days that will take you to 12 wks yourself peace!

Are the actual flowers turning brown? I almost thinking you might have russet mites! Pull a leaf and look underneath! They’ll look like slugs

Because you are growing at 24/0!!!!! All cannabis plants NEED sleep!!!!!!!!! Go to 18/6 in future and everything will be much better! Without sleep, cannabis CAN NOT finish its photosynthesis cycle and have a correctly functioning metabolism. A plant with an incorrectly functioning metabolism will NEVER grow properly. The MYTH that it is ok to grow cannabis for weeks/months without sleep needs to be removed from history ASAP! Your poor plants are dying from exhaustion.

Also, is there something to aerate that nutrient solution? There are situations you can get away without one, but it’s definitely recommended

My 2 cents to consider is salt build up in the medium, or other root related issues.

Hmmm. The gold leafs look to me like you are using reverse water???. Filtered water? If so its a boron issue. Switch back to tap and use that. Other than that. It'll be a lockout..too many cooks in kitchen. Locking you out. Good luck.

Roberts knows his way around coco - but I think your pH is ok, in fact it's perfect... I think what you've lost this grow to is powdery mildew ... Your humidity doesn't seem to be a problem - that's pretty perfect as well - which means there are spores lingering in your tent just waiting for the next victim. It's a nasty bit of work is powdery mildew... Before you try growing again, you've got to thoroughly and completely sterilize your tent ... get the bleach going and scrub everything including equipment, take apart your fans and clean them, clean your tray and valve, clean your pots... anywhere that spores could get into... I mean, do a DEEP clean/sterilization! This is so sad... that was a beautiful grow - and to think this is the fourth... my heart breaks for you! Good luck!

Looking at your diary you have a powdery mold issue. Reduce humidity is 1. The other reason your plants are failing is ph. I grow in coco. Your not matching the plants appetite at stage of growth. This causes the ph on coco to swing out of range causing issues. Test run off for ph. If over 6.2 there is your issue. If under 5.8 there is a issue. Coco grows have a limited window on ph. You cross the line and it takes less then a day to see signs. This is where I would start looking.