The first, obvious answer is that a 5.5 gallon fish tank is not even close to big enough for a GSP. They need 30 gallons each, since they grow to the size of a small football.
As far as how much salt to add, it depends on what you are doing. If you raise your level more than .002 per week, you'll kill your biofilter. It sounds like this is a brand new tank and fish, so you probably don't even have a biofilter established yet.
So, the first thing you should do is decide if you really want to keep this fish, and if you can give it the proper sized home. If so, look up at the top of our website at the "Learn" tab and read some of the articles in the library about keeping puffers, and keep reading here for how to mix the salt.
Anyways, you want marine salt, not aquarium salt. Make sure you have the right stuff, "aquarium salt" does not make brackish water.
Then, the best way to do it is to get a hydrometer that starts reading at 1.000 (freshwater), they are only a couple bucks and will tell you what your salinity is at (a refractometer is more accurate, but quite a bit more expensive). That way you can add some salt to your water and measure it, and see what you have. We cant' tell you "add X amount of salt to X amount of water to get X.XXX salinity" because it will be different for everyone depending on the humidity, water content in the salt, etc.
Generally a half cup of marine salt will make one gallon of water full seawater (1.022ish). So a quarter cup per gallon would give you in the area of 1.012, which is mid-brackish. An eighth cup of salt per gallon would give you right around 1.006, low brackish.
So lets say you want to bring your salinity to 1.002 the first week, in a 5.5 gallon tank you'd probably want to take a gallon or two of water, mix in somewhere around 1/4 cup of salt into it, then add that to your tank (take out some dirty water, replace it with the brackish water).. that *should* give you a little under 1.002 if I'm doing the math right. That's easy enough for the first week, but then when you want to raise it more, you need to do math and figure out how much to increase the salt to not bump it over 1.004 next time... this is why its easier to just get a hydrometer and double check your measurements.
You can always just do a tablespoon this week, two tablespoons next week, three tablespoons the week after.. that's nice and slow and safe.. the problem is, you need to know where you are at so you know when to stop, so you need a measuring device sooner or later. You can't just keep adding more salt eternally
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). Today I might go buy another one because I want two!!!
