Well, I'm not yet sure how brackish this tank will be. I just set it up with FW in it with my seeded filter pad. It's going to be home for the Opae Ula shrimp that I plan to buy from CL. I asked him is salinity but he hasn't told me yet. I suspect he doesn't know? I won't know either when I get them because I don't have a refractometer... I'm just going buy marine salt and use my TDS meter to match his water. Hoping that is close enough for these "tough" shrimp? I'll bring a sample of this water to my LFS and see if they can tell me the salinity. I am curious about that. I'm hoping TDS and salinity are related by a constant value within the brackish range.
Hydrometers are notoriously inaccurate and can be a pain to use to check current readings in some nanosize tanks. If you do get one you'll want to check its readings against someones refractometer to let you know how accurate its reading.