I saw this system years ago. Itās partially what inspired my current system because when i have the funds, i can change it up at some point. However, when i put my system together my income was quite a bit higher than it is now. Now i am working on a budget trying to improve things as I can. As I understand it too, you need a pump thatās moving more like 250 liters per hour rather than 125.
This is a photo of one of the more established plants. After what i did this morning, it is showing pretty obvious signs of nutrient burn. Guess I should have listened to your advice about cutting the nutrients Larry 
So Iāve now doubled up the water cutting the nute in half. Also, I checked the roots, no smell and certainly no slime. My nute is brown so i think the color on the roots is merely nute stain.
Nutes dont turn roots brown. Healthy roots are always white. Any brown coloration is some form of bacteria or algae.
Brown roots are less than ideal, but - If there is no smell, or slime, and the roots are not mushy, I would not be too concerned. They should pull off with a crisp, sharp little pop, rather than just mushing apart like a super soggy spaghetti noddle.
That said, brown roots in your condition are probably weaker than ideal and will be more susceptible to less than ideal water or temperature conditions. Think of brown roots are a warning sign.
Do your best to keep water temps between 20-21C if at all possible and yes - lower concentrations are better for nutes 
and dont forget the PH meter
They are looking kind of sad.
Get that PH meter yet??? 
We are going out of town next week. Thatās when I can get a meter finally. And that little thing is the auto that didnāt root right, nothing i can do for it now, at least itāll be a tease someday
So yeah, went to a bigger town, nobody has a ph meter, just strips. Trying to sort out something from ebay now cause amazon is buggy as all hell, only gives 7 pages of results.
If nothing else, get some drops. Strips suck, but drops will at least get you in the ball park.
Check the PH range the drops are designed to read. Some kits are for reading higher PH levels and some for lower. You want the lower range ones.
This one for example reads from 6.0-7.6. Thats too hi for hydro.
Look for a kit thats reads the lower ranges you are interested in - 5.0 - 6.0
Here is one example.
Iāve found a pen made by the same people as the one you linked. Just waiting to make payment now I know the water is high PH. RO water starts out too high for the plants and thereās chlorine added as well so raises it again. Also, amazon charges us shipping and other crap cause of where we live. Things go from 20 USD to 50 really fast. Ebay doesnāt care we not in a major center
Got the ph meter. It was somewhere around 7, didnāt really wait for it to stabilize when it was obviously high. Itās around 5.6 now. Hopefully thatāll stop the plants all wanting to up and die on me.
I think with proper ph youāll see a big difference. How often do you change your res? It can be wasteful but doing it more often can make things much easier.
Iāve been doing it weekly. I donāt believe iāll see proper results till i bud up these plants and start with some fresh ones in a couple of months.
Do you have any PH Down to lower the PH?
Phosphoric acid is the most popular thing to use, but you can also use several other products.
Next best is plain old battery acid - sulfuric acid. You can get about 1 quart from any auto parts dealer for under $10.
Take a one gallon jug and fill it about 3/4 full of water. RO or distilled is best, but tap will do fine. Then slooooooly poor in 1 cup of the battery acid. Then top it off with more water. That will give you a solution that will drop PH about like commercial PH down.
By the way never never never never never poor water into un-diluted acid. It can flash boil and splatter acid everywhere - which is bad, Very bad,
Other, somewhat less effective options are citric acid or distilled vinegar. Even regular vinegar will work, but the distilled stuff is much better.
My point is - if you drop the PH down now, the plants will start to recover now. No need to wait till next grow. You should see major improvements within a few days at most if you can get the PH down into the 5.5-5.8 range and keep it there.
I have ph down. I am keeping it around 5.6. A lot of the roots donāt look so great and the bigger plants canāt be pulled out anymore. I think the high temp for so long allowed bacteria to take place.
Try running a sterile rez. Add a little pool shock or even some bleach. The chlorine will kill off any pathogens but it will also kill any bennies. Plants use chlorine as a micro nutrient. Obviously you only use a really small amount. It allows your solution to be a higher temperature than normal also 
I think this is the first time Iāve seen weed grown in NFTā¦?
Very interesting.
Also great discussion and suggestions! 