| | Re: 55 gal filter
Get the FX5 going even on this tank. By the time you are ready to move up to a larger tank you will know if the FX5 will do what you want, and it will be well cycled.
Over 900 gph (per manufacturer) sounds like a lot for a 55 gallon, but note that is without media. 600 gph w/ media is not too much for a 55. Just about right IMO.
If you think the FX5 will do what you want on the 90 gallon, and you like the Fluval product, then you might want the same manufacturer but smaller for the 55. (I do not know what they are currently selling)
I run the Rena Filstar XP3 (a canister) on several 45 gallon tanks, and the Aquaclear 110 (HOB) on some. Either of these would be a good choice.
If you want less gallons per hour, then the smaller versions of these (XP2 or Aquaclear 70) are available, but I run these on smaller tanks.