We're worried about our 10gal filter being too strong?

We just replaced our entire filter because it was doing a crappy filtration job, and we got a tetra whisper for up to 20gal. a href="http://www.petsmart.com/product/index.jsp?productId=3065443" rel="nofollow"http://www.petsmart.com/product/index.js…/a sort of like that one. We have one molly (aggressive) in the tank, and it seems like she has to fight a tad harder to swim. Is there any way to keep the current less?