leaves burning and im stuck big time

hi people, i have 5 plants 3rd week of flowering and they are showing signs of major burn all over.
im using a rdwc system in 20 litre buckets.ive been struggling with these symptoms for a while now on a smaller scale ie just the odd few leaves here n there every few days,but it seems to be getting worse now..
ive flushed the system many times over last couple of weeks.now my levels are 600ppm on 1/4 nutes and ph of 6. the leaves are turning yellow and just burning up n dying.i feel the ppms are ok or even to low and the ph is constant 5.8-6. the light is a 600w hps about 2 feet above the highest plant. the only issue im having at the moment with this uk heat wave is temps ,in the tent its showing max temps of 95 degrees.i have no light leaks.
what are the symtoms of low nutrient levels? as throughout the grow ppms have not been above 800.or is it just plain old heat killing my babies,any help apreciated

Mg def.
Add a Ca/Mg or Epsom salts.
Epsom at tsp per gallon of res. Dissolve in water first and be sure to pH the res again.
Ca/Mg at 5ml per gallon of res..
I would prefer the Ca/Mg.

could it be due to not enough nutrients? i feel ive been over cautious this bein my second dwc grow and on the first i was was over feeding. i decided to give another 1/4 dose of nutrients canna flores/rhiztonic/cannazyme/boost & cal/mag+ .
my water here is 350 ppm out of the tap so 1/4 nutes is around 250/300 ppm .it was at 600/650 so now with another 1/4 shot of nutes it should be in the region of 900 ppm.

Mg def. Increase Mg.
Your waters ppm at the tap is BAD news ! It is part of the problem,,,,you don't KNOW whats IN that 350 ppm!!!
You will have FAR better success with them BELOW 150 ppm!!!

That's not an option really , that's the general standard of tap water here in the uk. Never had a problem in soil , never once had a nute defiency , it's only since I've swapped over to dwc due to it bein a lot more convenient for me that all these deficiency are popping up. My first attempt I had issues at first with over firt and this time I went the other way bein over care full I think. I no quite a few growers and no one uses ro water or anything like that in the uk because our water here is generally a lot better quality than the us etc . It varies from 250-350 and bottled water is like 150 ppm so I'd say it's not that much of a biggie personally . I've dosed these bad boys up now so hopefully they should perk up over the next week or so

3rd week

distilled water is ~5ppm can be bought at any supermarket in the USA< not sure about UK. If you cannot afford a RO filter. But i understand not wanting to buy bottled water for a plant. There are also places in America u can fill up 5gal water barrels with dist water for about $2, again no idea in UK.

I get were your coming from but like I say I've never had any issues with water quality , I'm not saying I'm a pro grower but been growing in 10 ltr soil bags for last 3 years and I've never had an issues , I never even bothered to ph my water i just mixed It up poured it in and I was growing 35 x plants that can't of been luck . A few friends grow hydro tables etc and they say the same the water is fine here , even my grow shop guy says this ain't the us so don't worry lol. It's not that I can't afford it I just think it's unesasery for ro or distilled water , that's my personel opinion , each to their own

Ok, that's fine. Do what works for you!
But, that's still a BAD case of Mg def.

Our water is worse in the U.S.???
I live very rural. My well water runs at 144 - 148 seasonally. I still RO for end quality.

Looks more like a Potassium deficiency too me.

It's prob a defiency in everything lol running 1100 ppm now and can notice the difference in one day

It's well documented that the water quality generally in the us is very poor . I no when we come over we do t drink your tap water because it makes us sick , If your well water is 140 ppm it's better than evian bottled water lol you should sell that stuff and your prob in the minority and I would not waste your money on ro 100% man that's just anal !