IMO, the carbon might really help, for exactly the reason stated by gillianandjoe.
As far as using tapwater & conditioner goes, whether or not it will work depends largely on where you live. The chemistry of tapwater varies A LOT from place to place, but generally it's not fit for use in a reef -- again, there are definitely exceptions. Where I live, the water typically has a fair amount of calcium in it, but it comes out of the tap with Nitrates of 20. :eek1:
FWIW,
Mariner