Natural seawater with low salinity?

[TikiBird]

Since I set up my reef tank 8 months ago, I have used natural seawater from a pet store close to my house.

I have done some partial water changes in the last month without testing the salinity of the store's seawater. (I know, it was stupid. It has always tested at 1.025 and I got complacent about testing it.) I was surprised when I tested my tank water this week and its salinity is at 1.021!

I got new seawater today. This time I tested it before dumping it in (sigh) and it's only 1.020.

I don't understand what's going on. If it's water from the ocean, as the store's fish manager says, how is the salinity so low? Is there anything I can do to remedy it?

 

[TikiBird]

Adding: I calibrated the refractometer with the solution it came with every time before testing, so I don't think my refractometer is off.

 

[Randy Holmes-Farley Verified Community Expert]

Salinity varies a lot in coastal regions due to rain and runoff and rivers. It could easily be that value locally, or they may be diluting it to save money.

 

[Randy Holmes-Farley Verified Community Expert]

For example:

https://www.researchgate.net/figure...-field-off-the-coasts-of-New-England-and-Nova

FIGURE 3. Mean spring surface salinity field off the coasts of New England and Nova Scotia derived from the Northeast Fisheries Science Center Oceanography Database (Fratantoni et al., 2013). The black dashed line denotes the NECAN study area. Low-salinity waters are supplied from the Scotian Shelf into the Gulf of Maine where they are further freshened along the Maine Coastal Current by numerous river and estuary inputs (black arrows).

 

[TikiBird]

Salinity varies a lot in coastal regions due to rain and runoff and rivers. It could easily be that value locally, or they may be diluting it to save money.

Ooh interesting points. Thank you, @Randy Holmes-Farley!