They actually don't need brackish or salt water. It is hard to get them through the larval stage, though.
My ghost shrimp set-up was a 10 gallon with a sponge filter. It had a coupla pieces of driftwood, was overgrown with algae and had a good population of copepods. I started with 20 shrimp. Despite the females berrying up constantly, I only ever saw maybe three out of each 20 eggs as free-swimming larvae, and only one out of 20 made it to adulthood. I've read that the adults will eat the larvae and I didn't separate the females when they dropped, so I'm assumin' that's a big reason why I didn't have a higher survival rate. My colony died when I was on a business trip, so I didn't have a chance to experiment more and I haven't replaced them yet.
They're interestin' critters. Good luck!
WYite