Do you have powerheads in the tank? Do you have a lot of surface aggitation? This, along with skimming, assists in gas exchange. This is a place to start. Do you remember if your pH was low before you added the airstones?
The best thing for salinity is a refractometer. They are superior to hydrometers which, though cheaper, tend to be less accurate. If you are never doing a reef tank then they may be tolerable, but if you do wish to keep various inverts, a refractometer is the better choice. I would find one online.