Difference between Refractometer and Specific Gravity Tester?

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You do not need both. As stated refractometers are much more accurate and a must have IMO for anyone with a SW tank.
Sisserydoo said:
Unless it is for a brackish setup, then the swing arm is fine!

Not for me I'm afraid or any other serious reef keeper with thousands invested in livestock/coral.

I have a red sea swing arm that I had when I first started, i like to run my tanks at 1.026, my D-D refractometer allows me to do so.

The swing arm reads 1.023......

Not accurate.
I don't think anyone is going to have thousands invested in livestock/coral running a brackish tank... not to mention, since brackish environments fluctate in salinity several times a day depending on the tide, accuracy isn't nearly as important.

I will say that I tested an old hydrometer today against my refractometer and got the same result... 3 times... but, that doesn't mean I'd recommend them as I've heard so many times not to trust them.
As I say, it's down to the individual.

You trust two pieces of plastic or a scientific instrument, your call.
I agree with everyone here. I got a hydrometer when I first started not knowing any better and I went to have my water tested just to make sure my readings were correct and it turns out my salinity was at 1.026 when my hydrometer said 1.023. Take it for what it is but in my opinion just save yourself the hassle and get the refractometer
I recall a time when I tested a water sample with a refractometer (the instrument was calibrated weekly with 53.0 mS/1.0264 SG calibration fluid) and it registered a reading of 1.016 SG...a swing arm hydrometer measured the same water at 1.022 SG. Prior to the refractometer's reading, the owner of this water sample and hydrometer couldn't fathom why the only corals that didn't die in his tank were mushrooms.
Hydrometer readings are very much influenced by temp. It's an extra variable. I'd use one for a mudskipper tank, but not a true SW tank (reef or FOWLR). All comes down to personally accepted risk / accepted effort.
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