I have a question when adding aquarium salt do u add how much your tank is,because it says every 5 gallons so i was thinking do i add 15 teaspoons for a 75 gallon.
1) What kind of fish in tank (some fish are not salt tolerent and die with any salt)
2) What is filtration
3) How long has tank been up
4) what is the tank water chemistry...house water chemistry
5) What is wrong that you want to add salt
Your calculations are correct for 1 tablespoon per 5 gallons of water...Just remember that if you don't fill the tank to the very top edge then don't use 75g in your calc's...If you don't already have it here's a helpful calc link...http://www.drhelm.com/aquarium/areajava.html