If you're willing to remove and replace half the water in the tank every week, then get a single filter that will turn over 4 times the water volume of your tank and keep a backup running. A 55 G will need a filter system with a gallon per hour (gph
) rating of a bit more than 200.
A couple of Hagen AC50s, about $38.00 each at the local pet store, set on opposite sides of the tank will work fine. If one goes south on you, you'll have a back up that will handle the gas exchange needed to maintain the tank.
The water change is the real filter, not the filters themselves. The filters do a good job of getting oxygen into the water, which allows carbon dioxide to escape, but do little to keep the tank water pure.