I never had luck with my hydrometers
I got rid of them 2 weeks after starting my tank and replaced them with this.
http://www.jlaquatics.com/phpstore/s..._ID=vt-refract
While you're at JL you may want to take note that they're test kits and Seachem products (prime,stability ) are the cheapest. Even with shipping
which you don't have to worry about most of the time because it's free with a 100.00 in product.
Just for giggles compare two hydrometer readings from 2 different units with the same water.
you'll see why they're not popular.