The only way to tell the true hardness of your water is to buy a carbonate hardness (Kh) test kit at the pet store. But it sounds like you do have hard water anyway.
To turn it into brackish water, all you need to do is buy marine salts that are used for salt water aquariums and add it at 1/4 to 1/3 the dose of marine (You should use a hygrometer to measure specific gravity but as long as you're consistant, that's okay). I used 1 cup of
Instant Ocean per 5 gallons of water with my mollies.
Make sure you dissolve the salt in water before you add it to the aquarium. Obviously they are in freshwater now, so the change should be slow. Also remember that the salt is always there... meaning that if you lose some water to evaporation, there is still the same amount of salt there. So to stay constant with the salinity, you need to top up with freshwater before you do a water change.
I hope I haven't made this too confusing
. What size tank is it anyway?