sry for double post, but i got 2 more tiger barbs today to make my school a school of 6. Luckily the all get along REALLY great. As soon as i took them out of the bag, the other 4 swam over to make the new 2 part of the group. The have been swimming together now. The old 4 of my tank seem happier now and all of them are swimming around alot more now.
Rice,
The real question here is, what are the water stats? I know that this is a relatively new tank, and it probably has not cycled yet. If you are not familier with cycling, please read about it in the pinned topics or search google for "cycling fishtank" and there will be many, many good articles that explain it. In a very quick nutshell, ammonia is excreted form your fish and you have to cultivate a large enough colony of ammonia-eating bacteria to remove all the ammonia every day. The big problem with ammonia is it is poisonous to your fish. In high enough concentrations, it will kill them, exposure to even a little ammonia will weaken their immune systems and make them more vulnerable to diseases.
The moral of the story is that fishkeeping is really about well over 95%
water-keeping. Keep your water clean, and your fish will reward you with long, happy lifes.
If you do not have a water test kit (most serious fishkeepers consider it an essential tool as it is the only way to know exactly what is happening in your tank), take a sample of your tank water to your nearby LFS. Any LFS worth anything will test your water for you.
Until you can get the ammonia concentration to zero, and then the nitrite concentration to zero, the fish are in danger.
When you get your test results, post them here and the members can help advise you on the best next step to try to make sure that no more of your fish die. It is important to note that the very first thing anyone is going to ask to help you diagnose a problem in your fishtank should be what are the water stats: ammonia, nitrite, nitrate for sure, and maybe pH, GH, KH.
Oh, and the green on the black's of the tiger barbs is just about how the light reflects of the colored cells of the fish. It is called the Tyndall effect after the physicist who first described it. If just the blacks are changed, it is just a light effect, but any change in the orange color may very well be a sign of something else, like stress. Stress possibly caused by poor water, and I cannot urge you enough to buy your own test kits so you know exactly what the condition of your tank water is.