A refractometer is beyond useful. That fast, accurate test for a qt transfer or salt mix is sanity-saving and fish-saving. And they're cheap. I got mine online, oh, years ago. The poor thing is afflicted with rust here and there and I never have had to calibrate it, not from the day I got it. You want the one for 'aquarium', not home brewing, and it will serve you well. My previous one finally rusted the plate-pin off and I decided holding it place with my finger probably deserved treating myself to a new one. Which I did. Came in perfectly calibrated five years ago, I've tested it now and again since then, and I just treated myself to a new bottle of calibration fluid. Still dead on. Trust me. It's an important purchase, and one that can give you fast, accurate answers. Those of you who wear glasses, you don't have to struggle: it's got a focus ring, like a telescope, and you can get crystal clear results.
