Depends on what type of livestock you have in your tank. With SPS you would want some pretty good flow but not overkill, If you have LPS SOFTies you want Just moderate flow nothing Crazy. for a 55 depending on DIMS i would go with 2 550s on each side. or a 750 and 550 just enough so you can get some nice movement and keep the water going.
Thanx very much..... I'm still new and don't really know all the names of coral or anemones so still learning but I will follow your advice any idea where to get it at a good price
i wouldnt get those i had them on my old tank after a while the impeller goes in reverse when you turn them on and off. go with the tunzes lot more features and a lot more money. but you get what you pay for