Salts are nothing more than Ionic compounds and different kinds of salts are found in a variety of freshwater ecosystems. NaCl is the most common in nature, but other salts occur do to erosion and ground water release. Blue Springs in Florida is an interesting example. The salts that come out of the spring are not NaCl, (and I couldn't tell you specifically they are), but due to there presence they harbor an diverse array of both fresh and marine species. Tarpon, Red drum, snook, flounder, ladyfish and mullet are seen along side plecos, pacus, tilapia, and native freshwater fish like gar, bowfish, bass, and other sunfish. This alone suggests that these fresh and marine fish are osmoregulating with different salts.
Salinity reports measuring the exact compositions of salts can be found in a lot of scientific literature. Perhaps reading up on those, and making your own formula could be beneficial, but like I said most fish have an incredible ability to osmoregulate. That's why tilapia are so invasive. They can survive extreme salinity, low oxygen levels, and have a wide temperate range.
