Yes, I'm pretty certain that's root rot but get some more opinions before proceeding with my recommendations.
In my opinion, if the plants are young then I would start over after thoroughly sterilizing the environment...otherwise, you'll probably be fighting this the entire grow. Pay particular attention to preventative measures with your next grow.
If you want to try to save these plants you'll will want to completely clean out your system. You should remove each plant and rinse off the roots. You should trim off any dead roots or damaged roots. Dip the roots in a hydrogen peroxide solution - 1/2 cup of 3% H2O2 per gallon of water.
You should then clean your entire system using a strong bleach or h202 solution. Soak everything (except the plants) in bleach for a few hours. Make sure you rinse everything really good before putting the plants back in.
Are you using organic nutrients? If so, you might want to run the nutrient solution at a lower concentration than usual for a while and throw in an enzyme additive like Hygrozime and a beneficial microorganism product like sub-culture (though the beneficial stuff is expensive). Don't use H2O2 in your nutrient solution with organics. You may want to lay off the organics until the plants recover.
If you're using chemical nutrients then the idea is to maintain a sterile environment. Go with a lower nute concentration mixed with 1/4 - 1/2 cup/gallon of 3% H2O2. The H2O2 dissipates, so you'll have to add some back with your top-off water. You'll probably have to do this the entire grow, because no matter how well you sterilize the pythium fungus will always be present in the roots.
You will also need to do some preventative measures. The colder (down to like 66* F) and more aerated you can keep the res water, the harder it is for the pythium to survive. Monitor your ph closely...wild swings, especially upwards, probably mean the nasties are proliferating again. Be ready to clean out your res several times if you can't keep it cool.
Hope this helps...