07-16-2010, 05:22 AM
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There might be some salt in all setups, but even extremely hard fresh water (i.e. with a lot of dissolved salts, but not necessarily sodium chloride) will still not even register on a refractometer. It takes quite a bit of salt to reach into truly brackish territory.
The issue is that, with many brackish species, their bodies are designed to live in fresh water when young and saltier water as they age. In nature, the adults spawn in fresh water and the young slowly move out to saltier environs as they age. Naturally, since the adults do have to swim back into fresh water to spawn, brackish water fish have a great deal of rapid adaptability to different salinity levels. However, for the long-term health of the fish, it's best to try to replicate what they would live in in nature. For example, Figure 8 puffers kept in moderately brackish water grow larger and live considerably longer than those kept in fresh water, which reflects the lifestyle of the fish in the wild.
Also, most substrates are inert and won't affect salinity. Some are calcium-based (crushed coral and aragonite sand) designed for use as substrate for African rift lake cichlids or saltwater, but these won't make your water more salty.
As for how to set up a brackish tank...well, it depends on how brackish you're talking about. A tank that will ultimately be only low end or moderately brackish could be set up very similarly to a freshwater tank, with rocks, driftwood and plants like java moss, java fern and mangroves. High-end brackish would be different still, and many "brackish" fish do just fine in full marine conditions as adults so at that stage the tank could be converted to a more typical FOWLR type setup.