Refractometer out of whack?

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I've received three separate orders of fish/corals where the salinity, as measured on my refractometer, is out of normal range. I don't know if the problem lies with the vendors or with my refractometer.

The first item came in water that I was told was 1.025, I measured it at 1.021

The second order came in water I measured at 1.028, don't know what the vendor's salinity measurement was.

The third order the fish water measured 1.021 which the vendor stated was correct, the invertebrate water measured 1.030 but the vendor claimed 1.025

Anway I did have two animals die withing a couple days of receiving them and I have no idea whether the salinity difference was a factor.

Anyway I need to cross-check my refractometer against other people's measurements. The calibration on the refractometer lets you zero it against RO water but there is no way to make sure the measurement of the tank water (1.025) is correct.

Have other people had trouble with either their refractometer giving false values, or livestock arriving in water that is way out of normal range?