NaCl has no place in your freshwater tank. the addition of NaCl to your system can wreak havoc on your fish, some species more than others. fish such as sailfin mollies will show no effects because they have adapted to life in saltier water then other fish. corydoras or pleco species, however, do not appreciate any added NaCl to their water at all; most Amazonian fish and African Rift Lake fish do not like salt in their water for long periods of time either.