Tsunami and I both have these on own 10g and it produces good current(might be even too much), we just stuffed it with filter floss and didn't use any of the filter media that came with it. I love this filter, the water is so clear. But I would still try a EHEIM classic on your 20g.
Here is a link to a good review of this filter: link
I, too, believe that this filter, although great for tanks between 5 and 10 gallons, is too small for a 20 gallon. Even if one relies solely on the plants for the biological filtration, I don't think that this filter would provide sufficient circulation in a 20 gallon tank.