If the tank is set as FW, you raise the specific gravity to the desired light brackish water (1.003 - 1.005) most safely in two steps, but should allow at least three or four day between increases. You also need to keep the bacteria fed during that time.
The fish can be drip-acclimated in a couple of hours maximum with no issues.
You might get away with going from S.G 1.000 to light BW in one step, but if the water is soft and acid (KH less than 3, pH less than 7.0), you may osmotic shock the osmotic shock the bacteria. Two steps is much safer.
Where's the fish? - Neptune