I think there is confusion:
- There is premixed salt water from the LFS (still synthetic but not from their fish systems, either): use a heater, and adjust salinity etc to your tank
- Many are now selling actual sea water in gallons - use a heater, and may need to adjust salinity, etc to your tank
- Then there is the water from an animal purchase - properly acclimate animal and dispose of water. Do not add to tank.