I use Seachem Stability for cycling and water changes. I have salt and freshwater tanks and this stuff works for both. You can use the media you have but do as Diana suggested and run a fishless cycle with the new salinity levels you desire so your bacteria has a chance to get used to it. Brackish fish are fun, what were you planning on keeping? I used to have a pair of Orange Chromides and a Knight Goby a while ago.