The tang will still swim around, but if it's in a tank that's too small, it will get ich. The minimum size for a baby yellow tang is generally considered to be a 55 gallon tank. They will outgrow a 55 though, and for an adult yellow tang the minimum tank size required is considered to be a 75 gallon tank.
Your tank is just too small for that fish.
About your goby, if you have added all of those fish within 3 months of setting your tank up, it's likely that you went too quickly. If you rush adding animals, then the fish often die.