leaves on my plant are rusting

my leaves are rusting on the on the edges then yellows out and dies. i know some yellowing is norm but my girl looks un happy. 2 plants first one is 4 weeks flowering 9L buket. 2nd one much bigger 2 weeks flowering 5gallon bucket but the rusting is just started. here is a pic
i had to use neem oil to rid of tiny tiny black spider bugs only visable with 30x magnifing glass that was a week ago when i chek under leaves i see little bugs but they are dead not moving at all any more and i dont see any spider web anymore. could it be because of that? or nute problem? so if anyone can help me out i apreciate it. thanx in advance.

im using:
pro mix soil
a+b nutes every 2 or 3 time i water
400 watt hps

Looks like a potassium deficiency. 1 table spoon of Epsom salt to 4 litres of water will fix it up

Ya Epson.. But looks like nute burn as well. Leave that plant alone for a while.. It doesn't like you anymore!!

it breaks my heart when u say that.lol. so dont give the epsom yet? because it seems like getting worse daily

How are you measuring PH? I would bet it's a PH problem rather than a deficiency.
Number 1. Promix is considered "soilless" which means your waterings and feeds need to be in the Hydro PH range 5.5-6.0 with 5.8 being ideal.
If you are using regular water, and nutrient solution without adjusting PH then that is most definitely your problem. You've got major nutrient lockout. What's the PH of your waterings, and feedings?
What's the runoff PH?
I think you'll find your answer there.
Get a PH meter.
Hope this helps.
Peace.

my water ph is at 5.5-6 i dont know what my run off is what should it be at you think? i chek ph of water then i add nutes then water is that not correct? should i chek ph again after nutes or just chek run off ph?

Nutes lower ph so after adding nutes let water rest an hour then check ph

You have a combination of problems. I think your PH is to low. This is giving you lockout / salt build up. It looks like you might be watering to much also. I see spider mite damage too, and they look like they might have some heat stress (could be wrong about that). Do you have a fan in there room? Do you know what temp it stays at when the lights on? How far away is the light?

I think your main problem is nute lockout from low PH, and to much water tho.

Well, everyone seems to have a solution for ya. Now you just have to decide who to listen to, lol.
Cut your feedings to 1/2 strength, and remember to ph your nutrient solution After adding your nutrients. It's kind of hard to overwater a soilless (really you can't) so I don't think it's that. I would keep a regular water/feed/water/feed schedule. Feed one day, and PH'd water the next. Check the runoff, and if it's not between 5.6 to 6 my advice would be to raise or lower the ph of your watering, and feeds until your runoff is within that range.

omg i dont know what to try from all these sollutions....

i tried epsom salt and light off 24 hrs still looking same.
i do have a fan and an egsahst in and out leading outside

How's it coming along? Any improvement? Hope all is good.

nothing changed much so i flushed it today hope it helps.
i been reading and i notised my leaves are curling downwards. and i gatta say its my 3rd grow and i never new to chek the ph of my water before and after i add nutes, i only used to chek it before when i fill up my bucket. so that where i am now. i flushed it with ph water and we shal see feels like it a good bye letter. i wish there was a doc i could call..lol

It seems when mites get to a plant as bad as they did yours it sometimes hurts them beyond repair. I hate mites, just hate them little fuckers with every ounce of my body.

It's all good man. It'll take a few days for things to turn around after the flush. Give em half strength nutes. Any damaged leaves will NOT repair themselves, but the new growth will come in right, so don't look for the old leaves to change.