[Scuba Kid]

Well for a brackish tank, you will need marine salt and a hydrometer to measure the specific gravity. This is why you don't listen to petco people.

The amount of salt will depend greatly on what you wish to keep. IMO, brackish water has the ability to vary more greatly than freshwater as the amount of salt can change to house what you are looking for, and in the wild the amount of salt fluctuates daily. First of all, a 29 gallon is not large enough for a full grown green spotted puffer and they do not usually tolerate tankmates. On top of this, they are full marine fish as adults. I don't suggest a GSP for the tank setup you have in mind.
As for plants, you will need to look for some that will live in brackish water.
Check this link out: http://www.wetwebmedia.com/BrackishS...bracplants.htm

Here's a great article with info on brackish fish. Read through the different species for ideas on what will work in your setup as well as what interests you. http://www.wetwebmedia.com/BrackishS...rlvstk_sel.htm

Good luck.