tankmates for very small tangs

Guppies/ mollies, a trio should work out.

1 trio and then your tank will be stocked. Reg 30g don't offer much surface area for bottom dwelling fish like tangs, so you are completely limited to top dwelling fish.
 
My tangs are kind of quiet fish, I don't know if they'd appreciate the boisterous movement. Plus, the rainbowfish will take away from your juli, calvus, and compricisseps. I really recommend guppies (feeder males are better), they are good dithers and don't take away from your fish.

The only other thing I can think of would be danios...
 
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