# Can Someone Explain Percentage Salinity to Me in Comparison to Specific Gravity?

Zach72202
Member
So here's my dilemma. I am reading scientific research articles about breeding pufferfish - specifically T. biocellatus, and they speak of keeping the fish in 20% salinity and various other % of salinity, but I have no idea what this means. I am familiar with specific gravity, but I am just curious what the conversion from % to sg is. I cannot find it online--or at least in terms I can understand online. Can someone please explain this to me!

Frank the Fish guy
Member
Take a look at the hydrometer ()

You can measure the density of the water directly and compare to the density of water. That is the specific gravity.

Or, if we know that the extra density is due to salt, then we can compute the equivalent amount of salt needed to raise the density. That is given as parts per thousand (ppt)

Sea water is around 1.026 specific gravity, which is about 35 ppt. 35 ppt is the same as 3.5% salinity.

35/1000 = 3.5/100 = 3.5%

I doubt that they kept their puffer in 20% salinity. 20% would be nearly fully saturated solution (pickle juice) and would kill the fish. Saturated salt water is about 26%, or 260 ppt.

But a salinity of about 20 ppt, or 2.5% would be a brackish water condition the fish might see.

Zach72202
Member
Frank the Fish guy said:
Take a look at the hydrometer ()

You can measure the density of the water directly and compare to the density of water. That is the specific gravity.

Or, if we know that the extra density is due to salt, then we can compute the equivalent amount of salt needed to raise the density. That is given as parts per thousand (ppt)

Sea water is around 1.026 specific gravity, which is about 35 ppt. 35 ppt is the same as 3.5% salinity.

35/1000 = 3.5/100 = 3.5%

I doubt that they kept their puffer in 20% salinity. 20% would be nearly fully saturated solution (pickle juice) and would kill the fish. Saturated salt water is about 26%, or 260 ppt.

But a salinity of about 20 ppt, or 2.5% would be a brackish water condition the fish might see.
This makes a lot of sense and I didn't think about the hydrometer! I was riding in the car reading when I posted this, so I didn't have one in front of me! I appreciate you explaining this to me!

As for the puffers, I will just post an image of the study. I don't think it is farfetched. From what I know, I wouldn't use quite as high % sea water for raising the fish, but according to the results they spawned the puffers successfully so they did something right!

Frank the Fish guy
Member
That is not percent. Percent uses this symbol %.
Instead they are giving salinity as parts per thousand. This is called permil, instead of percent.
So they add an extra o on the bottom. 0/00

So they started the fish at 20 permil. That means 20 ppt.
Look at the hydrometer. 20 ppt is the same as a specific gravity of 1.014.

We scientist and engineers like to speak in jargon and special shorthand. Keeps the riff raff out.

Zach72202
Member
Frank the Fish guy said:
That is not percent. Percent uses this symbol %.
Instead they are giving salinity as parts per thousand. This is called permil, instead of percent.
So they add an extra o on the bottom. 0/00

So they started the fish at 20 permil. That means 20 ppt.
Look at the hydrometer. 20 ppt is the same as a specific gravity of 1.014.

We scientist and engineers like to speak in jargon and special shorthand. Keeps the riff raff out.
Ah yes the little circle anybody can miss because nobody but people who use it understand it! At least I know now lol! Thank you for the reply!

RayClem
Member
Many aquarists do measure salinity by measuring the specific gravity using a hydrometer such as the one from Instant Ocean. However, they are not always accurate. Salt deposits can build up on the float and the pivot can stick. Also air bubbles can accumulate on the float giving false readings. They are simple and quick, so they are useful. However, the most accurate way of measuring salinity is using a Brix refractometer. They were designed originally as a way of accurately measuring the sugar content during the winemaking process. However, they are just as effective for measuring salinity.

Thus, I suggest getting a hydrometer for routine salinity checks, but get a refractometer as well to confirm the accuracy of your hydrometer. The refractometer need not be expensive:

Amazon.com : brix refractometer

Jesterrace
Member
RayClem said:
Many aquarists do measure salinity by measuring the specific gravity using a hydrometer such as the one from Instant Ocean. However, they are not always accurate. Salt deposits can build up on the float and the pivot can stick. Also air bubbles can accumulate on the float giving false readings. They are simple and quick, so they are useful. However, the most accurate way of measuring salinity is using a Brix refractometer. They were designed originally as a way of accurately measuring the sugar content during the winemaking process. However, they are just as effective for measuring salinity.

Thus, I suggest getting a hydrometer for routine salinity checks, but get a refractometer as well to confirm the accuracy of your hydrometer. The refractometer need not be expensive:

Amazon.com : brix refractometer
Personally I would just skip the Hydrometer and just go with the Refractometer since they are generally more reliable and don't cost much more.