Like I said last night, my opinion is that the urban legend about a tank requiring 5x/hour water turnover is based on the assumption, usually well founded, that the tank will be stocked to a certain level (all the fish that will fit). If your filter is cycled and has demonstrated its ability to remove all the waste produced by your fry, the same filter should still be adequate for the same fish in a larger tank.
Surely there's some limit to that, but a 55 gallon tank isn't so much larger than the 20 that I would expect trouble. The only problem you really could have is an area of the tank that wasn't receiving circulation. I think if you place the filter 2/3 or 3/4 of the way toward one end of the tank and place the airstone at the other end you'll have enough water movement to prevent formation of a stagnant area with ammonia.