Hydrometers are inaccurate do to the fact the float could be effected by other things then salt, ie, bubbles or anything else that changes the density of water. If running a FOWLR tank, it is not horrible to use a hydrometer, and there really isn't much of a difference between brands, the biggest difference is some come with thermometers which could be useful. But if running a tank with more delicate inverts or extremely delicate fish, using a hydrometer is extremely risky and a refractometer would be a better option, you can get one on sale for around $30 if you're lucky.