I never saw those rust colored net pots before. Nice job. 
I used to use black ones, but switched to these rustas, as these also have an openings on the sides, not only at the bottom. I thought that it would be better for the root system:-) more freedom
Hi! My plants are : White Widow 1 – 15-17 days in veg; White Widow 2 – 7-10 days in veg, OGKush – 7-10 days in veg. No nutrients, but keep p.h. about 5,8 almost all the time (as i’d been told before). Is it time to begin with some nutrients? And how long should the plants stay on veg.period with adding nutrients??
Thanks for advance!
got p.h. jumped up to 8-8.2 
don’t know what to do, may someone helps?
I would think that the ph meter probe dried out and is no good. Check it against water you have with a known value. Get some PH solution, and check it out. When my values have skyrocketed in past years; It was general the probe. If you left the top off, and let the probe dry up…It is done
Do not add nutrients until you have 4-5 trues sets of leaves alternating on the main stem. You also want to see some roots poking into the reservoir. Seedlings will thrive in plain old water.
As far as rustas…Al my black net pots have slits all around. I believe you had just a pot. Net pots were developed for growing 'orchids", in case anyone is interested.
pot for orchids. I have grown in them my first two marijuana plants) 
Right now I have one WW with 4-5 sets of leaves alternating on the main stem, and OGK and one more WW which don’t have 4-5 sets of leaves yet. Which way is better for future blooming, wait and begin to add nutrients when all 3 plants will have sets of leaves? or it’s better to begin with nutrients for the biggest one for now, and later for those two plants??
I think you will be fine waiting a couple days until all are ready for nutrients.
Unfortunately i changed the solution of WW1 on 7th of December, and that’s what i get the next day(
and yesterday I poured all the solution with nutrients, and made a new solution without any nutrients, just reduced p.h. at 5.8. Know think, if the affected leaves should be pruned?
Looks like you might need more air at the roots and/or you need to keep the reservoir’s temperatures a little lower. That looks like it is root rot, it can also be caused from anaerobic conditions in the reservoir as well as pH or nutrient imbalances. It looks like you have the light blocked off from the reservoir, any light getting in can also cause algae to grow on the roots and kind of smother them, plus the algae competes for nutrients and oxygen and can contribute to anaerobic conditions leading to root rot.
Thank you for instructive response and support! As all my three plants were in the same conditions, and the problem touched only my WW1 plant (after i added to her the nutrients),i think it more looks like the high temperature in reservoir, bringing to the root rot or ph or nutrients imbalance. I can quite be wrong, but i think its most likely because of light getting in reservoir. As for the deficiency of air, I think less likely, because the last time, when I raised my two plants, the aeration stones in the reservoirs, were several times smaller than the ones I use now, nevertheless I could avoid such problems. To prevent entering the light into the reservoir, I poured some expanded clay into the pot, thereby closing the openings through which light can enter the reservoir and trigger process of root rot.
U hate to say it, but it appears you are in a bit of trouble. I agree with MacG. Looks to me like those roots are looking very unhealthy. Anytime I have had an issue in the res, and found roots with that type of appearance; The plant was pretty much done. Sorry
yes it’s very sad will it wither and die, or what? Looks like it continuous growing…
If you’ve changed out the reservoir and it is hanging in there, it might recover. You can also try adding a little h2o2, that might help. Hydrogen peroxide helps in cases like this, it can reduce the algae problem and adds much needed oxygen to the reservoir as well as killing other pathogens that might be on the roots. The drugstore stuff isn’t very strong but you can use it as well. They make a stronger version sold online, even specifically marketed towards hydroponics growing, you can usually find it at the usual places, e-bay or amazon, etc.