A single fancy goldfish in a 20 gallon hex?

I love goldfish, but don't have the setup for a comet or a Koi. I was thinking about getting a single Pearlscale and keeping him in an acrylic 20 gallon hexagon tank I have. I also don't have much experience with goldfish, so I'd like to know if this set up would work and what sort of special filtration I'd need, as I am aware they are messy fish.