changes should always be done slowly (especially with sea stars) but you can also increase sg
by alternatively topping off with well mixed SW
one day and FW
the next until you get your sg
where you want it. I'd aim for an sg
closer to 1.022-1.025 IMO
should be mixed 24+ hours in advance before adding to the tank preferably with a power head. The newly mixed SW
should match the ph/temp/sg
of the main tank as closely as possible to avoid osmotic shock to your tanks stock.
Swing arm hydrometers like yours are notorious for getting out of calibration (especially if ever dropped). I'm sure your sg
is lower if you have been taking out SW
and replacing with FW
but as already mentioned a refractometer
is much more accurate/reliable.