If it's a standard biocube,I would personally use the built in sump filter...
After my years and experiences, I decided that canister filters, for the most part, work pretty well. It's a question of what works best for you.
I mostly use Rena/API, recent Fluvals are good and much easier to prime than their older models, eheims are ok, but I really hate having to prime them... Anything from a recognized name should work well for you.
I imagine to use one you will have to either remove the lid or make holes for the hoses.. put the intake in a back compartment have the return in the main aquarium.
as for goldfish .. meh, adding a filter will take detritus from the water, it will not allow you to keep messuer fish unless you clean it often . Adding lots of plants either submersed or emersed will allow you to have more bioload.. but plants and goldfish do not always mux well
But the best of luck