Tetras don't tollerate salt at all! They are from the soft waterways of the Amazon Basin or from the Congo River Basin in Africa. These are old waterways and have a strong flow. Also, these fish live far up the waterway far from the coast. They have probably never seen an ounce of salt, and have adjusted accordingly through the years.
The swordtails, guppies, and mollies on the other hand are from river deltas and can tollerate low levels of salt. They are usually sold in FW and need to be acclimated to the desired salt level very slowly.
Also, you need to use marine salts not "Aquarium" salts. "Aquarium" salts are for medication not for sustaining fish in a healthy environment. The marine salts are manufactured to closely mimic saltwater. Mixing this at considerably lower levels with freshwater will give you the desired effect. Good luck and Good fishing!
