As stated above, a brackish tank is 'half saltwater, half fresh water'. More specifically, it is where the ocean and a river system meet. No matter what you look at to define it, the most improtant thing to being successful (IMO) is making sure that you get the specific gravity that you require for either plants or fish. SG ranges from 1.025 (Full saltwater) to 0 (technically) Freshwater. Different brackish fish live in different SG. Some require an SG change throughout their lives. Other than that, it is essentially FW with salt.
Irish