Hey all,
here are my tips to reduce/eliminate root rot: keep your water under 68 degrees, use black air tubing, make sure you don't have any light hitting your roots/water AND provide lots of oxygen in the tank. The last point is probably what saved my grow: I am using 4 gallon waterfarms and got a severe case of pythium when using a small air pump with only one air stone. I upgraded my system to a 60W 1100 gph air pump and now using 3 air stones. My reservoir is now a real root spa. This little change made such a huge difference: my plant went from drooping leaves to growing 1-2 inches per day within 72hrs. Some might say I am overkilling it with a commercial pump: you can never have too much O2! Have used molasse and several organic nutes and never had a case of root rot since then. I also recommend using beneficial bacteria (Aquashield, Voodoo Juice) and fungi (Piranha, Tarantule, Great White). This will protect your plant from future attacks.
Hope that helps!