With this you can see if your water is 99.9% pure. I'm reading 0ppm TDS
in my RODI
water, it means the water is so pure it does not conduct electricity.
And that's this level of purety you need in reef aquariums
I'll suggest you mix your own salt. I'm using RedSea salts, this is the best IMO
, params are all high. Make sure you add salt to the clean water, and not the inverse.
If you have FOWLR
tank, then go with cheaper salt like Fluval Sea Salts.
1.020 = fish only
1.023 = LPS
1.025 = SPS
Most hydrometers aren't accurate, so I keep mine at 1.023, but I know in real life it's 1.025, as somebody tested my water with a refractometer.