Amphipods should be fairly easy to see (up to about 1/4 inch). You probably got some on the Cheato -- wait a while, and you'll probably see them all over your 'fuge. Seems like it took a month or so. Give them some rocks and places to hide / breed in. Copepods are really tiny and harder to see. I also ended up with lots of mysid shrimp in the 'fuge, but didn't really notice them for a few months (I might have mistaken them for Amphipods, though). You just need to look at the 'fuge very closely -- best after the lights just come on and everything is out. I even recently discovered several baby brittle stars in there (about 1.5 inches across).