I like to think about where in the tank a fish will spend most of its time, and how it's going to make its living, as well as things like color and eventual size. In a 30, you'll probably want to avoid fish that grow too big - but there are a _lot_ of fish who'd be perfectly comfortable in a home that size.
There are a whole crew of gobies who'll share a burrow in the sandbed with a pistol shrimp - I've got yellow watchmen (the female is gray...) and blackray (also called red-banded or yellow-rose) shrimpgobies. Clown gobies and neon gobies will spend their time among the rocks - without shrimp. The clowns can be shy, but the neons (also the sharknose and yellowline gobies) will sit out in the open, offering their services as a parasite removal specialist.
You might take a look at a wrasse of the genus Wetmorella - possum wrasses are small and dainty - but I've never kept them myself. Keep in mind that wrasses, as well as firefish, are champion jumpers!
You've probably seen damselfish at your local fish shop, and goggled at their feisty attitudes and brilliant colors - leave most of them in the shop. An awful lot of them are cranky. There are a few in the genus Chrysiptera, though, which should get along in a tank the size of yours. Yellowtails come to mind, their brilliant blue with a yellow accent plays well against the oranges of the clowns, and if there are kids in your world, they may delight in the "baby Dory"! Even these guys can sometimes set up a territory, which they'll defend against all comers.
One of the smaller blennies, such as the tailspot blenny, should work well - but only one, as they can be territorial too. They'll also help a bit with algae in your tank.
What about a brilliant and stately flash of magenta and yellow? Royal grammas are small members of the sea-bass family who'll set up a home in a cave they like and generally keep watch just outside the front door.
Whatever you choose as your next fish, I hope you can find a place to put a separate tank as a quarantine. My first round of fish brought ich with them, and I've only one of those original crew left - a zebra dartfish. (Another peaceful choice, but a bit longer at about 4-5".) I've quarantined since then - though not always perfectly - and the fish in my display are healthy and sturdy.
Just a few thoughts to get you started. In the end, when you look into the tank, it's _you_ who's going to feel the peace and serenity of your reef - or the sadness if something's not quite right. Research and learn, because you and those fish will need to take care of each other!
~Bruce
mine got the easy life and didn't need to build a burrow, which would have been fun to watch, but my sand choice made it impossible to keep one without a ready made burrow, all he had to do was move in
