Its hard to answer that question directly without having more details.
If you keep certain riverine species cichlid such as Viejo maculacuada or the soda lake cichlid Alcolapia alcalicus, it would be nearly impossible to over salt. If you keep soft water rheophillic species like Rheoheros lentigenosus or a forest species like Betta albimarginatta or some inland tetra species, like cardinals' a sudden rise in osmotic pressure from a bag of salt in a small tank, could either directly do them in, or cause enough stress to cause a disease leading to death somewhere down the road.