Wow poor DP... This is why you should never listen to what the LFS
Yes there are many fish that live in brackish water, but are better kept in saltwater tanks. The reason being brackish fish is too costly. Think about the cost of salt for weekly water changes. If you can get the fish into saltwater, then you can add live rocks, skimmers, refugiums, and lots of other stuff to delay water changes.
This only applies to high-end brackish fish (ie green spotted puffers, ceylon puffers, monos, scats, some eels, etc). Don't convert low-end brackish fish into full salt.