Well, the pinned topics here will help. My own web site (see link below) has a fairly detailed FAQ on brackish water fishkeeping.
The short version is that brackish is not difficult at all, and in some ways easier, because the salt inhibits things like whitespot and fungus. Basically you need a packet of marine salt mix and a thing called a hydrometer (costs about £5-£20, depending on the type) or a refractometer (around £35+) to measure the density of the water. These will tell you how much salt is in the water. You want a density (called "specific gravity", or SG) of 1.005 for these puffers. When you do a water change, all you do is add some salt (about 8g/litre), stir for a bit, and then let sit for 20 minutes so that all the salt dissolves. Then you use your hydrometer to measure the density. The more salt, the higher the density. I use a very cheap and simple floating hydrometer similar to the type used in home brewing. Not especially accurate, but it does the job.
Once you have the salty water made up, you pour it into the tank. Do a water change of, say, 20% each week, and eventually you will convert your freshwater tank to brackish slowly enough not to stress the fish or filter.
Don't forget that should only top up any evaporation with freshwater, not brackish water.
Most plants don't like brackish, but some are fine. Again, there are pinned topics to help.
Beyond that, it's all the same as freshwater. When you buy new fish, adapt them carefully to the brackish water if they were sold as freshwater fish. Putting them in a bucket and dribbling in a little brackish water over an hour or so usually does the trick nicely.
Cheers,
Neale
thanks very much neale u'v helped me loads. Me and my partner keep fresh water tropical fish but know nothing about brackish water is there any extra equipment needed or is it just a case of maintaing the salt levels?
thank
Clare
