How salty should I make it?
Well, that depends on what you want to keep. Many brackish water fish will do fine in a SG as low as 1.005. This is considered low-end brackish. Many common fish such as most gobies and figure-8 puffers will do fine for the entirety of their lives at the low end of the spectrum. This is a bonus if you want to keep plants as well, since most plants that tolerate brackish only do so at the low end of the spectrum. Other species, such as Ceylon puffers and Columbian sharks, will require low to mid end brackish as juveniles and move on to high-end or even full marine as adults. The particular requirements of your specific livestock will help you determine what your target salinity should be. Research and double-check your information well before starting the tank to ensure that your species are compatible and that you are willing and able to provide the proper habitat.