So I need something that will eat most of my molly fry... suggestions?

I heard that mollies eat their own fry, but none of them have touched the new fry in my fishtank. Because I don't have room for all the fry that will be coming in the future (they multiply so rapidly) I was wondering what fish would live in my tank of mollies that will also eat fry? The full grown mollies aren't doing the job. I still want a couple or a few fry to survive, but I can't house this many when they are full grown! So I need a fish that will eat them but won't be aggresive to the full grown mollies. Can you give me some suggestions? I have a 20 galleon tank which has a little bit of salt in it and lots of decor/plants.