I had one but never could fine how it worked and sold it lol When i burly set up my tank. You think a ebay one will do it?Hydrometers are not accurate at all none of them.
I used 3 diff brand and type hydrometeres all said my salinity was at 1.024 when in actuality it was 1.030 and i was having the same problem with high alk once i got a refractometer and got it lowered to 1.025 my alk was norm at 9 afer a few water changes.
The more salt you use the more other added nutrients you get just my 2cents.

