From the brackish section.
What if I already have the fish?
This is a relatively common problem the well-meaning aquarist buys a freshwater fish from Wal-Mart or a large chain pet store, and after doing some research, finds out that the poor fish belongs in brackish water! Dont panic. The saving grace is that, in an estuarine environment, these fish have adapted to a certain amount of adversity in their conditions, so usually a short stint in plain freshwater will not kill them. You have made the important first step however, in identifying the requirements for the fish and wanting to correct them.
So, we will assume that the fish is home in your tank and acclimated to the situation for the time being. The next question is, what about the other fish in the tank? While brackish species will tolerate freshwater fairly well for a period of time, freshwater species unfortunately do not handle brackish as well. So, if you have a brackish species mixed in with freshwater, they will need separate habitats. At this point, you must make a decision about whether you have the time, space, resources, inclination etc. to devote a new tank to the fish in question. If the answer is yes, then great! If not, then you should probably think about returning the fish to the store if you are unable to properly care for it.
Now that you have the fish and have decided you want to keep it, lets move on to the next step. Get the fish set up wherever it will end up, whether thats the current tank or a new, cycled one. Now you can begin the process of converting it to brackish. The sensitive part of the system is not the fish at this point, but the bacteria in the filter. They are nowhere near as forgiving of salinity swings as the fish are. So, your goal is to slowly add salt to get the tank to the desired SG. The usual recommendation is raising the SG by .002 per week until you reach the desired SG. This will give the fish and the bacteria ample time to adjust to the change.
