Hi Mr. Goldfish,
I use Whisper Power Filters. I know there are much better filters out there but I started with them because they were easy to get around here and they are very economical. I don't have to worry about getting filters for them because they sell them at nnnnnn. I have found them to do a good job for me especially now since they do have the bio foam. But, I also keep the maintenance up on my tanks very regular.
For a 20 gal. tank, if I'm not mistaken you need at least 200 gph. That would be a minimum. More is better because goldfish are so messy. I know the 5-15 pumps 90 gph and I think the 10-20 pumps 100 gph. I am using a 30-60 on my 46 gal. tank, which is not really enough filtration according to goldfish standards, but it's bare bottom with only 2 fancy goldfish so it's doing a great job at the moment.
I would like to mention to you about the common goldfish. It is suggested that they have 15-20 gal. of water per fish. Because they will get very big, up to 12".
If at all possible you may want to consider getting a larger tank for them in order for them to live to their full potential. I have one common and they do get big fast.
But anyway, If you stay on top of your maintenance I think the whipsper's are fine but try to get one with more gph.