So I have a 20 gallon tank that Im either going to make a salt or brackish tank for a green spotted puffer or some marine fish. How does brackish differ from salt, do I use marine salt or aquarium salt and how much do I use per gallon? Also Id like to add a background to the tank, I plan on using paint, what kind of paint do you guys recommend and for a salt tank im thinking blue background, or should I just go with black?