I would skip the barbs in favor of a smaller, less feisty schooling fish, personally.
edit- oh,sorry- so you already have the barbs? are there eight of them? I'd say that's about stocked for a 30.
How about some kuhli or chain loaches for the bottom, or corydoras catfishes? I've always wanted chain loaches, but can never find them. I might be worried that the tiger barbs will be aggressive toward the rams and other fish. But they are in a good-sized group so that lessens their aggressiveness towards other fish and they will basically bicker among themselves.
I love Kribensis... they are very pretty and fun to watch! Especially when you give them a cave or house to set up shop in.
I've never been happy with tiger barbs in my experience. I love the look of them, but whenever I try them, they end up nipping the heck out of everything I have... including things that are supposed to be ok with them. But by all means, if you already have them, hopefully you'll have better luck than I have with them!
I also have a clown pleco who is just cute as a bug! :dance2:
Maybe some rainbow fish? I have a school of dwarf neon rainbows... very active and cool. I don't see why 8 tiger barbs, 2 kribs, 1 clown pleco and a school of 4-6 rainbows wouldn't work in a 30 gallon.
Oh yea, I meant dwarfs... I have 6 dwarfs, they only get to 2 inches. I wouldn't put any of the bigger rainbows in a 30 gallon.
I like the idea of a bottom feeder too. I have heard that Kribs don't like Cories but my Kribs don't even seem to notice my Cories (though my tank is a bit bigger too : )
Kuhli loaches might be cool... but I don't know how well they clean up. Cories do a great job, though!