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Provided you're a good and faithful water changer by replacing half the tank's volume every week or so, then a 55 G tank only needs a filter with a gph of a bit more than 200. If you tend to slack off on the water changes, then your fish will need all the added filtration you can give them.
What fish you like to keep
How many decorations you want to have
How heavily planted to want the tank
I find that 10x the tank volume per hour is just about perfect for filtration. You are going to want to shoot for something different if you have species that prefers a different amount of flow though.