adding salt

You shouldn't be adding salt water back into the tank for top offs, only fresh water. The salt doesn't evaportate with the water so if you top off with salt water you're raising your salinity each time you top off.

If you've been doing this for a while and your hydrometer hasn't shown the increase it's definitely time for a new one or even a refractometer.

I used a hydrometer on my 55 for years and my salt level stayed at 1.025. I finally broke down and spent the $40 on a temperature compensating refractometer and wouldn't you know it my salinity was at 1.030 which is way to high.
Out of curiousity I took my new refractometer to my friends house to test his water and his was way high as well...

Hydrometers are notoriously inaccurate esspecially as they get older. They also don't compensate for the temp of the water.

 

Wow cool! Know of a good, reasonably priced one?