You've got the Eclipse System 12, right (combined shallow bowfront acrylic tank with matching hood and lighting)?
The 12 flows a lot of water compared to its smaller siblings (Eclipse 3 and 6), to the tune of 150gph, which should be fine for this single fish...assuming he's not too big of a goldfish. In that case, 12gal would be too small. BTW, my little Eclipse System 3's flow a sedate 35gph.
If you add a canister filter, it had better be a small one, or you'll have a lot of current that the fish, with its problematic swim bladder, may be unable to handle.
I'll second TetraFreak's judgment on this one. Try it solo first.
If you have to get more filtration, use the existing cutout for the light & pump electrical cords to run some U-tubes for inlet and outlet to your selected add'l filtration choice.
$20 is a great price for this.
v/r, N-A