40 gallons is what I would consider to be in the nano range so I think you'd be looking at smaller fish.
An ocellaris clown, a fairy/flasher wrasse, a watchman goby, and maybe a small group of cardinals would work well I think.
Of course, and obligatory cleaner shrimp or pistol shrimp if you get a watchman.
Try to find fish who have very small territories in the ocean and aren't very powerful of swimmers, and at the same time won't rely heavily on natural live foods supplied by your tank (i.e. pods). This can be a difficult balance to strike.
Checking out the nano tank forum for ideas wouldn't be a bad idea! Happy reefing!
Cool fish for a 40 would include; swales basslet, yellow assessor, royal gramma, chalk bass, lantern basslet, midas blenny, stigmatura blenny, a group of barnacle blennies, a grammistes blenny, a group of zebra dartfish, a group of blue gudgeon dartfish, firefish, watchman goby, neon goby, sunburst anthias, yellowhead jawfish, purple tilefish.