I've been researching your problem.
although theres a couple of reasons why the tips of your roots may be turning black
(Magnesium overdose) I'm sure it's Root Rot.
http://www.ipmimages.org/browse/subimages.cfm?area=62⊂=6960
just like these guys said, you need to clean your system.
All municipal water systems with old piping have slime mold. H2O2 kills slime mold and lots of other stuff, including algae.I use 4 tbsp/gal of water and let is sit for 4 hrs before I use it with plants. 500-1000 ppm is the recommended concentration of 3% solution.
Look for the Hydrogen Peroxide thats 3%
Put your plant some place safe, and clean your system.
Clean
EVERTHING. If it can get wet clean it.
Run your pump or Bubbler for a while in your H2O2 solution.
Rinse everything really really good.
run your pump or bubbler for a while to let it rinse.
And then run a second rinse.
while thats running you need to take care of your baby.
cut the Rot off of your roots.
don't go nuts just cut the root back into the healthy white growth.
After your done with that.
WASH YOUR HANDS
you can spray off your roots with your H2O2 solution.
Spray them good, and spray them some more.
I think your problem is your water.
go and get some distilled water, give that a try.
or clean your tap water before you use it in your Hydro.
Every time you change your nutes clean your system.
You can now start off where you left off.