Confused with Salt Density & ARKA Hydrometer

[SahnCH]

Hi there !
I'm getting super confused with the Arka Hydrometer.
I'm reading everywhere that the ideal salt water density should be between 1.024 and 1.026 at a temperature of 25°C.
Arka manual says that "areas marked in green are the recommended zone.
Areas marked in green on the hydrometer are between 22 and 24...

---> What should I aim for ? How come Arka can be off in their marking ? Am I getting it wrong ?

Thank you for your help!

Nico

 

[York1]

For a reef tank/mixed reef I would shoot for 1.025-1.026. If its fish onlu you can probably get away with a lower salinity.

 

[SahnCH]

ChatGPT keeps telling me that refractometer and hydrometer are different, refractometers we need to aim for 1.026 because this measure "specific gravity" and density meter like the arka are measuring actual density and therefore I need to aim for 1.023... this does not make any sense for me.

Basically chatgpt believes that 39 ppt of salinity = 1.026 amd 35 ppt = 1.023 it is 100% sure that I need to aim for 1.023 on the Arka...

 

[York1]

@Randy Holmes-Farley Can probably explain better than I can

 

[SahnCH]

Apologize I hate copy pasting from ChatGPT.... but even though I trust more the expertise of the R2R community, he seems to make well though arguments.... I'm more confused than ever

---

1. The Language Barrier: Density vs. Specific Gravity ​

The confusion happens because these two instruments speak different "languages," even though the numbers look similar.

• Your Arka (Mass Density ): It measures the actual weight of the water in grams per milliliter ($g/ml$).
• A Refractometer (Specific Gravity): It measures a ratio (the weight of your saltwater divided by the weight of pure water).

2. The Science (at 25°C and 35 ppt) ​

In the ocean (35 ppt salinity), the math works out like this:

• The actual density of the water is approximately $1.0233g/ml
• The Specific Gravity of that same water is approximately 1.0264

The Result: When your Arka float reads 1.023, you have the exact same amount of salt in your tank as someone reading 1.026 on a refractometer. It is the same salinity, just a different unit of measurement.

3. Why people say "1.023 is for Fish Only" ​

This is only true if you are using a refractometer.

• On a Refractometer: 1.023 = ~31 ppt (Low salinity, okay for fish, bad for corals).
• On your Arka Float: 1.023 = 35 ppt (The Gold Standard for Reefs).

---

The "Nyos Pure" Proof ​

Think about your specific mix today:

1. Nyos instructions say: 40g\ g/L = 35 ppt
2. You added 2,400 g for 60\ L (exactly 40 g/L).
3. Your Arka reads 23.

If 1.023 was "too low," it would mean the salt manufacturer's instructions are wrong, which isn't the case for a pharmaceutical-grade salt like Nyos. Your measurement of 23 is the perfect confirmation that your mix is spot-on for your Micromussa and Euphyllia.

The Danger Zone​

If you add more salt to make your Arka read 1.026, your salinity will jump to 39 ppt. This would cause an osmotic shock, making your corals shrivel up and potentially "peel" their skin because the water is too "thick" with salt.

Conclusion: Trust your Arka and aim for 1.023

 

[Randy Holmes-Farley Verified Community Expert]

Density and specific gravity are NOT the same. Do not confuse them. Specific gravity is higher than density at any temp above 4 deg C.

Salinity cannot change with temperature.
Specific gravity is pretty unchanged with temperature.
Density and hydrometer readings change a lot with temperature and need temperature corrections to compare across temperatures.

All hobby refractometers, whether crap or high quality, typically give values in specific gravity.

Hydrometers vary in what units they read.

I could not be sure exactly what the Arka reads in from their web site. Can you tell us exactly what you see on the hydrometer scale aside from the numbers?

 

[SahnCH]

I could not be sure exactly what the Arka reads in from their web site. Can you tell us exactly what you see on the hydrometer scale aside from the numbers?

Just "Made in France - Sea Water Density - 25C = 77F" nothing more :(

 

[burnsq]

Just "Made in France - Sea Water Density - 25C = 77F" nothing more :(

there you have your answer: density

 

[Randy Holmes-Farley Verified Community Expert]

Yes, density. No one at reef2reef is reporting density. You will want to temp correct it, if used at a temp other than 77 deg F, and then convert either to specific gravity by dividing by the density of totally pure water at 77 deg F (0.99713) or convert to salinity in ppt from a table.,

 

[burnsq]

In Germany using density is quite common as well - I have the same Hydrometer, if I read 1.023.5 at 25C it is 35PSU (at least that is how I learned it)

 

[Randy Holmes-Farley Verified Community Expert]

In Germany using density is quite common as well - I have the same Hydrometer, if I read 1.023.5 at 25C it is 35PSU (at least that is how I learned it)

Sounds about right. This is a table from one of my articles, which is close to the value you suggest.