For a 20 gallon tank I am very fond of Marineland Emperor 280s. They are a good workhorse of a filter having plenty of room of ceramic media, a cartridge you can change (I blast mine clean and keep them a LONG time) and a biowheel. They give you 280gph - just about perfect for a correctly stocked 20 gallon goldie tank.
You can shop on line - but recently, I have found that PetSmart on line has them the cheapest around here (cheaper than BigAls!) . Print off the page on the online site and take it to your store and they will match the price.
You should have an air feature, too, I feel. A simple air pump with the air bubbler of your choice - disk, stone, bar, etc. is fun and a good insurance policy against higher temperatures and stagnant portions of your tank.
Congratulations! I LOVE the potential of a new tank. SO fun!!!!