It's really very simple. Make up the right salinity in your bucket of water, and add that to the aquarium. Don't add salt directly to the tank, and don't try and change the salinity all at once.
Example: Suppose you have a freshwater tank you want to make brackish, to SG 1.005. On the first weekend, you do your 25% water change, replacing old freshwater with a bucket or two of the water at SG 1.005. When you check your aquarium, you'll find the specific gravity is only around 1.001, but that's fine. On the next weekend, you do the same thing, adding new buckets of water at SG 1.005. When you check the specific gravity of the tank, you find its only about 1.002 or 1.003. Again, that's fine.
That's it! Within a month or so you should find the specific gravity is precisely where you want it. Subsequent water changes won't change the salinity, because the water removed and the water added will have the same salinity.
Brackish water fish are intrinsically tolerant of salinity variation and indeed freshwater conditions for short periods. So there's no rush to change the salinity. If you suddenly changed the salinity, things like filter bacteria and plants may be shocked. Your fish couldn't care less about the salinity change, but they will care about a crashed filter! (This said, going from 1.000 to 1.005 in one fell swoop doesn't normally cause problems with bacteria.)
Cheers, Neale
