Cyprinids, (Loaches Barbs etc), and Characins, (Tetras etc), evolved in salt free environments. To them salt is a pollutant.
Some species can tolerate small quantities of salt, but generally even these live shorter lives. The fishes kidney has to work hard to keep the blood/eletrolytes balanced - this extra workload causes it to fail much sooner then it otherwise would, and when that happens, the fish dies.
Loaches, (and other scaleless fish), are much more affected by salt then others - ion exchange through the skin.
I am not aware of any small commonly available Cyprinid or Characin that will tolerate brackish conditions. There are some large Cyprinids that frequent the lower reaches of rivers and venture into the saltier brackish environs from time to time, (Some Tor sp. for example), but even the smallest of these are half metre fish.
The comment about the shoaling habits and your tank size is also valid. In a tank that size even with optimum water for this type of fish, you are not going to be able to keep many.