Man I've been using an Instant Ocean hydrometer for the past 6 years and haven't had any problems. Really all you're testing for is consistency, you just want your water change water to match the tank water. Does it really matter how accurate it is other than when you first fill the tank?
I know my hydrometer is off by .004, tested it against a refractometer. Easily enough I filled it with ro water and sure enough it read 1.004. Corals are a hell of a lot tougher than what you guys think in my opinion, think about when it rains in a tidal pool or even when corals are exposed on a low tide there's gotta be huge salinity swings.