Nuke
Has your tank cycled? Go to the Krib and learn what you can about the nitrogen cycle. Barbs are not exactly aggressive but they are very "pesky" they chase and they nip. Theyset up definate pecking orders and it's possible that your little one has been picked on and is quite unhappy about being in a little tank full of bullies.
On the other hand if the tank hasnt fully cycled the fish could be exposed to toxic stuff in the water. Barbs arepretty hardy but maybe the little one is the fist to show symptoms. Barbs are very active and often dart around the tank alot. I hope some others come along with some more ideas. My advise would be wait and watch.
Ask your LFS to test your water. With a new tank I wouldn't be to quick to do a water change unless the test indicates it . If you know someone with an established tank you could get a handful of gravel from them to seed some beneficial bacteria and jump start the nitrogen cycle. If you tank has cycled you might consider adding one or two more small barbs to take some heat of the other one. Most people say you need 6 or more TBs for them to be happy.
One more hint, even if a tank has cycled, you shoul introduce you fish to the tank slowly- especally on a small tank. This is defenitely one of the harder things to learn as a beginner. Patience, patience, patience. It pays in the long run. Your more likely to stick with the hobby too. It is addicting. Remember it's supposed to be fun so relax, and take it slow. ENJOY
