Question about Dragon Fish?

Are you talking about an arowana or a dragon goby?

If you're talking about an arowana, they are certainly not brackish, so I'm guessing it was a dragon goby you picked up. Without more information though, I'm not sure I can help you.

Edit: Okay, a dragon goby. I can help you now.

1. What types of cichlids are you planning on keeping? There aren't many that are saltwater. The best tankmates for a dragon goby are Colombian sharks (if you have the space), and other gobies such as the bumblebee, candy striped, knight, etc.

2. You will only be able to sex your goby if it is old enough. Look at the urogenital papilla which is located in the ventral region between the anal opening and the anal fin on both genders. It is short, blunt and yellow in the females, and it is thin and pointed in males.

3. There is no set lifespan because they are very delicate. I'd say ten years or more in impecable conditions.

4. You should get one of those devices that measure the salinity of your water (sorry, the name has escaped me), which only cost about $10 at your local fish store. You should fill the tank and cycle it as you would a freshwater setup, then buy aquarium salt and add the proper amount and measure the salinity everyday.

Be sure not to add more salt when you do water changes or top the water up because it is only the water evaporating, not the salt. Also make sure not to let the water evaporate too much because the less water in the tank, the more salty it is, which can be harmful to your fish.