I just wanted to make sure that everyone who has questions about salt and those that use it and may or may not know this information had it readily available. More specific information is welcome so please add it if you have it. Salinity ranges for live bearers would be helpful.
First and foremost, only add Aquarium salt, sea salt or pickling salt. Never use standard table salt. Never use any salt with anything like iodine or anything else added to it. I have also heard that rock salt, due to the possibility of more imputirties, is not a good idea to use.
When you add salt, regardless of how careful you think you are, there is always a chance of adding too much. With a water change you try to add the right amount for what you took out but it is almost impossible to get it perfect. I used to use salt in my tanks for the obvious purpose of preventing Ich and it did work. I also realized that I needed to get a hydrometer because I had no clue what my "actual" salt levels were. When I did I was actually about 45% higher than I should have been. You can be close, you might be right on but keeping the levels constant and within a certain range of salinity will not only keep things balanced but will keep your fish happy. Also remember that at different times of the year the amount of dissolved salt could change in your tap water especially if you are on a well or on a public water system that is not treated or goes through R/O like mine doesn't.
If you are going to use salt as a regular regiment, get a hydrometer and know where it should be for the fish you keep. There is no better way to keep proper track of the levels and to provide the fish with the best and most stable environment you can.