100 lbs for a 125 is fine, but it may be on the slim side if you want to rely on the LR
for biological filtration. For that, you need 1.5-2 pounds per gallon to be effective. I would suggest getting 150-200 pounds if you can afford it, so you can add it at once to cycle your tank.
Fiji rock is very colorfol, and will come with coraline growing on it, depending on where you buy it from. www.liverocks.com
is awesome, I'm sure everyone will suggest buying from them. The cheap stuff might not be fully cycled, depending on where you buy it from.
I would highly suggest you pick up a RO
unit if you can swing it... but if not, definately buy the LFS
if it's RO
. Don't use tap. Either way, it should be the same... just a good investment to have your own unit so you can make pure water from home.