Help! Hydrometer way off, salinity was low.

[MNscoe]

I’m six months into my first tank, being careful but still having newbie problems.

After initial calibration at purchase, I’ve been checking and zeroing my hydrometer on RODi water regularly. Today I thought “I should check with the 35ppm calibration fluid I got six months ago.” After I zero’d on RODI it read the 35ppm fluid at 40ppm!! (Did multiple tries.) Stunned, I tried my cheapo swing-arm density tester, sure enough it read 31ppm. See pics.
Calibrated the hydrometer to 35ppm and my tank water indeed reads 31ppm, agreeing with the float tester.

So: accepting that I’ve been running at 31ppm for prob a few months, what’s the recommended rate and process to bring up to my goal of 34ppm? How quickly or slowly should I do it? Any other remedies I need to consider for my tank after this period of low salinity?
Thanks!

 

[Fish Fan]

I'm sorry you're having some frustrations!

First thing is that most members here do not trust those swing arm hydrometers, I would personally not use one of these (and I've had several in the distant past).

Likewise, many times the salinity calibration fluid you purchase may not be right at 35 ppm. I'd suggest using Randy Holmes-Farley's instructions for making your own 35 ppm standard solution using everyday Morton's Iodized Table Salt found at any supermarket, and a decent kitchen gram scale. Use this solution to test your refractometer before making any changes to your system:

Reef Aquarium Salinity: DIY Calibration Standards

Reef Aquarium Salinity: DIY Calibration Standards By Randy Holmes-Farley One of the most important issues facing marine aquarists is providing a suitable environment for their aquaria's inhabitants. Among the important properties for a marine...

www.reef2reef.com

 

[BryanM]

If you do find you need to raise it (I would definitely re-verify your salinity first) many people just put mixed salt water in their ATO. This effectively slowly raises your salinity, which is what you want - slow.

 

[Randy Holmes-Farley]

I'm a little confused on what you actually did. Is there a typo in this sentence?

After initial calibration at purchase, I’ve been checking and zeroing my hydrometer on RODi water regularly. Today I thought “I should check with the 35ppm calibration fluid I got six months ago.” After I zero’d on RODI it read the 35ppm fluid at 40ppm!! (Did multiple tries.) Stunned, I tried my cheapo swing-arm density tester, sure enough it read 31ppm. See pics.

Do you really mean "I’ve been checking and zeroing my refractometer on RODi water regularly."?

 

[exnisstech]

I check the calibration of my refracter weekly. I was using the diy fluid posted above and it worked well but I have since started using a TM hydrometer when mixing my new saltwater to 1.025 at 77 degrees and check calibration of my refractometers using that fresh salt mix.
I don't remember ever seeing a refractometer that used rodi to calibrate except veegee brand IIRC

EDIT I agree with above in regards to topping off with saltwater until yu reach the salinty you want then switch back to rodi.

 

[MNscoe]

I'm a little confused on what you actually did. Is there a typo in this sentence?

After initial calibration at purchase, I’ve been checking and zeroing my hydrometer on RODi water regularly. Today I thought “I should check with the 35ppm calibration fluid I got six months ago.” After I zero’d on RODI it read the 35ppm fluid at 40ppm!! (Did multiple tries.) Stunned, I tried my cheapo swing-arm density tester, sure enough it read 31ppm. See pics.

Do you really mean "I’ve been checking and zeroing my refractometer on RODi water regularly."?

Yes, I thought that consistently maintaining zero point on the refractometer using RODI would keep the refractometer calibrated. Apparently that is not true.

 

[MNscoe]

I'm sorry you're having some frustrations!

First thing is that most members here do not trust those swing arm hydrometers, I would personally not use one of these (and I've had several in the distant past).

Likewise, many times the salinity calibration fluid you purchase may not be right at 35 ppm. I'd suggest using Randy Holmes-Farley's instructions for making your own 35 ppm standard solution using everyday Morton's Iodized Table Salt found at any supermarket, and a decent kitchen gram scale. Use this solution to test your refractometer before making any changes to your system:

Reef Aquarium Salinity: DIY Calibration Standards

Reef Aquarium Salinity: DIY Calibration Standards By Randy Holmes-Farley One of the most important issues facing marine aquarists is providing a suitable environment for their aquaria's inhabitants. Among the important properties for a marine...

www.reef2reef.com

This is a great article, thanks. I will make some DIY calibration fluid.

Given that my refractometer (when calibrated on the store fluid) and my swing-arm tester are both showing 31ppm, it’s pretty likely the store fluid is ok and that my tank is indeed low. I will slowly ATO it back up.

 

[Fish Fan]

This is a great article, thanks. I will make some DIY calibration fluid.

Given that my refractometer (when calibrated on the store fluid) and my swing-arm tester are both showing 31ppm, it’s pretty likely the store fluid is ok and that my tank is indeed low. I will slowly ATO it back up.

If you find that your sanity is truly low, raise that slowly. You can much more safely lower salinity quickly, but to raise it you want to go slowly

 

[Randy Holmes-Farley]

Yes, I thought that consistently maintaining zero point on the refractometer using RODI would keep the refractometer calibrated. Apparently that is not true.

Using ro/di is inherently inaccurate for many types of refractometers when used for seawater (even if they say to do so), and even for those where it is not necessarily inaccurate, calibrating anything far from the range of measurement can introduce new errors.