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I had two Emperor 400's on my 55 gal Cichlid tank, and it looked great. I guess it depends on the fish load you are going to keep. I had 14-18 fish in my tank, though.
1 400 will do fine for a 55G, but I also recommend adding a powerhead on the end of the tank that will have poor circulation. I have 2 400's on my 55G, and it's great, good flow on both ends of the tank, and no need for a powerhead.
Or if you want to do something a little cheaper, a 400 on one end and a 280 on the other.