Hmmm, my copepods started showing up about 2 weeks after I put the LR in. Then the amphipods were crawling all over the place about 2 months after the good LR was put in the tank. Maybe you'll want to get a little LR from a tank that has copepods, mine just came from our local LFS. I've never bought copepods, but if you end up getting that mandarin you may have to do that. A 20 gal just won't do for making enough copepods for the mandarin to eat.
I don't know anything about leaf fish, I was wondering if maybe your tank inhabitants were eating all your pods, but I don't know what those guys eat!