Aquarium Illusions helped me figure out my hydrometer thing... Just take it (them) in there and ask to check it out based on their refractometer!

are you taking at least 5 scoops before a reading? Nearly always, on a first, or second dip you'll have a tiny lil microbubble that's attatched itself to the swingarm (Even the smallest lil bubble will make a massive difference.)