Water conditioner should always
be used even if you're using salt already, and to be honest I'm not sure if there is a 'best' out there, I do know what I like however. :)
I personally like using NovAqua Plus
(with Echinacea, Vitamins and Anti-Virus Protection) for my water conditioner and AmQuel Plus
which removes Nitrate, Nitrite, Ammonia, Chloramines and Chlorine, and toxic organics found in older water. I like to 'premake' my water with NovAqua, AmQuel, and salt in a water jug for when it's needed later. I've been using these products for several years now.
These were suggested http://www.bettatalk.com/water.htm
I know others prefer using Prime
which is also an excellent choice.