They sell water that is right at 1.025 for calibration. You can adjust the screw to make sure it's set proper. I too was unaware and ran into issues of too much salinity.
If you have a gram scale you can make your own saltwater test solution. You can buy a bottle of it you want, but it's easy to make. Now some people will say you need salt mix that you would use to add water back in if you're doing a water change, not really. So here it is, this is a weight to weigh formula , it makes a 100 milliliter amount. 3.65 grams of salt (I used table salt) and 96.35 grams of RODI water. Mix together until clear. This will give a reading of 1.025 when reading through the refractometer. Make sure you zero it with RODI water then take your reading. I'm assuming that your refractometer is one that automatically adjusts for temperature? Most that I've seen are. Also that it's one for saltwater and not for 'brine' solutions, there is a difference. When you get your reading and it's not 1.025, then you'll need to take note to what it is and know that you'll have to adjust how you read the levels when doing tests. There are several articles that have been posted here on how refractometers can be 'off ' it's not always linear. Good luck, I enjoyed learning about my refractometer, it reads spot on, so I know that I'm making good quality saltwater and my levels in the tanks is good.
