My plants are in the second week of vegetative in a hydroponic system, I am using 600HPS light 18 hours a day. my water tank holds about 13 Gallon. but the roots dont look good, they have some dark spots as you can see in the pictures, leafs have some brownish spots, and they look kind of weak starting from yesterday, also leafs sides are kind of bending starting from yesterday as you can see in the picture. please let me know how to stope these problems and grow them the right way. also just to make sure please advise me how much nutrients exactly should i mix with the 13 Gallon of water? how far away should the light be from the plants?
ILGM Support Ticket:
What is the strain and type(unknown bag seed, strain name, regular seeds, feminized seeds, auto-flower, etc)?
Indoor or Outdoor? If outdoor, planted in ground or in a container?
Size of space (max height and area, length/width)?
Soil or Hydro? Type of Medium used? System type?
pH? Of the soil or medium (root zone/reservoir/run-off) and of the water and/or nutrient mix that is fed to the plant?
Type and strength of nutrients used? NPK? EC/TDS/PPM levels? Of what you feed it as well as what is in the water by itself and in the soil/medium (run-off), or reservoir.
Temperature? Day vs. night temp or highest and lowest temps? Root zone temps?
Humidity %? Day vs. night
Light system/watts/lumens/FLUX/PAR?
Ventilation system? Size? CFM? CO2? AC, Humidifier, De-humidifier?
Number “weeks/days” from into Season, Vegetative Growth or Bloom/flowering?
Add anything else you feel would help us give you a most informed answer. Feel free to elaborate, but try to be brief and to the point. Short and to the point questions and/or facts will help us help you 
What’s your res temps? it looks to me like start of root rot when’s the last time you flushed res? what are you using for nutes?
Temperature is 92 , I flushed the water last week.
I added 10 galon of water 5 of it with the below:
B.C Boost 72.2ml / B.C Grow 36.1ml /Thrive Alive B-1 Red 11.4ml / MagiCal 11.4ml / SugarDaddy 47.5ml . Last night I added another 5 galon with the same amount of nutrients. I’m mentining the pH around 6.5. By the way I’m using haydropnic
If that is your res Temps you are in trouble res temps should be 66-72f
How to reduce it - since they are in closed closet.
I’m using 600 HPS.
Hydro ph 5.5-5.8 is ideals res temps mid 66-72 also you don’t want any light penetrating your res so cover ever empty holes. To reduce temp of water place a ice pack in it
I’m talking about the closet temperature not the water. Because from the pic I’m guessing I have heatstress & root issue on the same time. I’ll check the water temperature anyway
you say closed space how big? do you have your light in a cool tube with venting and fan? Fill out as many questions on the ticket as you can the more we know the better the advice will be thnx
nice looking system seems better designed for a 400 watt than a 600 can you control intake and exhaust rates? 90f is very hot without adding CO2 and if the temp hits 90s the res will get very hot too. This is the source of your heat stress and root issues I would use a 600 watt in a 48x48 grow tent but not much smaller space without having full environmental control the heat they throw off makes it tricky to keep temps right in so small a space.
So you think the problem with our root and leaves mainly because of the heat if I replace it to 400 how long until I see improvement.
Root rot is a pest you will have to get it managed cut off any infected roots then flush system using h202 peroxide to eliminate remaining bacteria
http://www.cannabis.info/gb/abc/10005474-the-marvels-of-silicon
you will want to add some root promoters to recover faster as well
As for the light if you can’t speed up exchange rates to get environment cooler yes use smaller light. You will have to do your homework on use of H2o2 and root rot control I’m sure others will have some suggestions for you as well but now you know the cause and the what make things simpler. 
Should I cut off the damage root completely, isn’t going to effect the plant!! - since I don’t have H2o2 and it will take time to order it. Shall I flash the whole recivor and install a fresh nutrients is it going to be enough.
To give you an idea in my 4x4x7 tent the air temp would stay 76-80 daytime so I didn’t think the water temps would be very high… I am running 2, 5gallon, DWC buckets and decided to check the rest temp and it was at 78f. To back up what Donald said, check your res temps! I ended up buying a couple of water chillers just to keep my temp in check, the high water temps were causing brown slime (bacteria I didn’t want lol) to form in the water. Since chilling i havent had that issue.
Edit: autocorrected res to response… Hate autocorrect at times!
http://www.quickgrow.com/gardening_articles/hydrogen_peroxide_horticulture.html
H2o2 (peroxide) they are the same thing
Don’t cut them off! Once you get a handle on the rot new roots will grow from them, you will see that they are a nice pearly white or creamy white color depending on age. Go to your closest store and buy a big bottle of hydrogen peroxide, at Walmart we got one for around $1. Put some in the res and let it do its magic. I don’t suggest using it all the time, but rather get some beneficial bacteria like botanicare Hydroguard or some Great White.
read the article and relax I am simply trying to send you in the right direction the more you learn about the better your future grows will be and the less mistakes you will make. You will be able to save these plants and adjustments to any set up are to be expected.