Did you get the Penguin 200B then (200 gph
)? That's actually the one I would have picked for a 20 gallon. All the tanks I have that use HOB
filters have at least 10x turnover per hour. IMO
the manufacturer's rating for HOBs is usually very overestimated; like if you really put that filter on a 50 gallon tank there would be a bunch of dead spots and it just wouldn't be powerful enough. If you raise the water level in your tank, that might help with the surface agitation. You shouldn't have any problems unless you are keeping fish that really don't like a lot of current, like bettas. I'm not familiar with the newer line of Penguins, but the older model Penguin I had allowed the user to adjust the flow by pulling up on the intake tube. Maybe that is possible for yours as well.