Join Date: Apr 2003
Location: Mid-Hudson Valley, New York
I don't think there is an ideal, but you do have to have some general hardness. Plants use both calcium and magnesium. My water has almost zero hardness out of the tap, and I add Seachem Equilibrium, which supplies both of those ions as well as potassium. Some folks add CaCO3 and Epsom salts instead. I like to have at least 4 or 5 degrees GH.
I use Flourish for micros and the same ferts you would find at Greg Watson's site for macros. Since you're in Oz and I'm in the States, I think I should let someone else give you specific recommendations on that.