I keep hydroguard around always, although I don't always use it. I bleach everything, tubing to tub lids, between grows.. Once you get rid of the black roots, even hydroguard may not be enough to fix it. This problem is normaly due to organics mixed in the rez for long periods and slime becomes home for bacterial colonies. These colonies directly effect the nute mixture and pH balance in the rez. First, cut out any black or mushy roots and remove the material as fast as possible. Put the plant in a plain water bucket while you address the rest of the equipment. Bleach the shit out of all of it and run some bleach water through your pumps if applicable. Then check your root mass again, and make sure it's as gone as can be and reset the system with a fresh batch of nutes.