Most people I know who run with a hydro have water that's off by .003-.005. A refractometer is a necessary purchase in the hobby, IMO
That looks like the refractometer that I bought. Make sure whatever one you get has automatic temperature compensation. The SG
is temp dependant and it would be a pain to have to calculate that on the fly if you needed to be ultra accurate (like for hypo).
Join the NJ Reef Club
Today! Saving the world's reefs one living room at a time!
Atlantic City, NJ September 25-27, 2009