howdy jack - I know its your first post but it didnt belong in the intro section so I moved it into the infirmary for you
there are a few hydro products you can try - hydrofungacide or pythoff or you can use peroxide - I will let somebody else chime in and give you dosing on that and it depends on what % peroxide you can get
you need to figure out what caused the rot as well - not knowing anything about your system (besides its e/f) my first thought is over watering
need more info - how often do you feed and for how long - what are your water temps - what nutrients/additives?