Well based on the type of environment you are deciding to create it will depend on the amount of flow you'll need. 170 gph is to low for any of those solutions for a 55 gal however.
FO (Fish Only) you should have 10x the tank gallons in turn over or 10 x 55 = 550 gph
FOWLR (Fish only with Live Rock) 15 to 20x or 825 to 1100 gph
Reef (any type of corals) 25x and up (I've seen some at 70x) 1375 gph or higher