Tangs should only be introduced to tanks that have been established for a while. At least 6-8 months, but a year is better. You should do exactly what yetti said. Just top off with normal saltwater instead of freshwater. When water evaporates, the salt doesn't evaporate with it, so adding saltwater will really increase the SG. do it just a little at a time. you should raise it to about 1.025 over the period of like a week or so.