Fw has a SG of 1.000, SW has a SG of 1.020-1.025 (normally). When creating a Brackish tank, you need to SLOWLY bring the SG up to 1.014 (is the level I recommend).
Meaning you start at 1.001, and slowly bring it up from there. It should take you about two weeks or more to get to 1.014.
You need to have a hydrometer to know what SG you're at.
Try to re-read every single thing I've posted, think about it for awhile, and try to figure out how to do it properly.... It's really not that complicated
