Mr. Saltwater Tank TV Friday Am Quick Tip #71: I’m Accurate As Long As You Keep Me Tuned Up


Don’t forget to make this piece of equipment is dialed-in and spot on

Browse the Store! Questions?

Comments for this article (34)

  • Bob says:

    Not earth shattering 😉 but a good important tip to know!

  • Matt says:

    Maybe not earth shattering but certainly very, very sensible… How many people actually do it? I imagine not an awful lot…

    Reminds me… I haven’t done mine in a while… job for tonight!

    The calibration fluid… I hear it gives you a better calibration result… any truth in this do we know?

    Thanks Ray and thank you Mark 😉

  • Michael says:

    Basically calibration fluid is a solution of known specific gravity. As long as you immediately replace the lid once you’re done using it, it will stay constant in that value. I believe there are different calibration fluid out there set to different specific gravities. The one closest to what you want your specific gravity to be would be the best one to use since that is going to deviate the least from your target. The one I use is for 1.026.

  • Matt McDaniels says:

    I calibrated my refractometer using RO/DI rather than calibration fluid. I ordered calibration fluid on a whem… just because it was cheap and I was getting such much stuff already (this was after my tank was a year old). My refractometer had been reading 1.025 the whole time, but was off .005 the whole time! For an entire year my tank had been running 1.021. Don’t use RO/DI, the fluid is cheap… just buy it! Haha

  • Mike C says:

    I calabrate mine every time before I use it.
    It takes 45 seconds and is worth the piece of mind knowing
    It’s correct.

  • Matt K says:

    This is excellent advice. The past 6 months I had watched the corals in my tank fade in color. Everything was looking terrible. I couldn’t figure out what was wrong. All parameters seemed to be ‘spot-on’. I performed water changes religiously. My TDS meter read 000 ppm. I was at a loss. Until I decided to check the calibration on my refractometer. Using RO/DI water, the original calibration wasn’t off by much. So I ordered some calibration fluid. Whoa! Accurately calibrated, it showed the salinity in my tank had dropped to 1.018! No wonder the corals weren’t happy.
    Over the next few weeks I brought the salinity back up to 1.025, and the color in the corals returned immediately.
    I should have checked it sooner!

  • Mad Hatter's Reef says:

    Great Tip Mark. Keep them coming!

  • Jon says:

    Just bought mine from Marine Depot. Thanks for the tip.

  • stel1os72 says:

    Now THAT’S what I call perfect timing Mark! My refractometer was delivered just last night from BRS…WITH calibration fluid.

    …thank you too Ray, for reminding us all about an easy chore we too often take for granted. Like Mike C. said, it only takes about a minute for the piece of mind of knowing your tank inhabitants are safe.

  • Eric says:

    A little late for me! I traded in a puffer two days ago.(small fish were disappearing at night) the store owner told me my salinity was “way high!” I took a look into his refractometer and it was 1.031 holy smokes I went home and calibrated with RO/DI slowly started diluting considering I had just returned with $225 worth of fish and inverts.

  • karen says:

    Good reminder ~ Thanks Mark! Just calibrated mine this morning!

  • Matt says:

    Michael – Just like to say thank you for the clear answer as its very much appreciated… 😉 Calibration fluid isn’t talked about much over in the UK unless its for a PH probe!

    I’ll be picking some up this weekend!

    Some cracking info folks… Such a simple thing but lots of comments coming across… 🙂

  • Paco says:

    So dont use RODI water??? cause i just panic i thought i had my tank read at 1.024 since i started and i just now calibrated with RODI and im running at 1.021 …

  • jhunsbar says:

    Is it that important to use the tool and isnt a manual tester good enough. I cant believe that the salinity count can be that far off or am I that wrong. My local people tell me its all because people on the internet want to sell the refractometer and its not that critical..

  • Alexander says:

    I would also warn against using RODI after calibration fluid I was reading 1.022 instead of 1.026 since like 95% of refractometers, mine was a NaCl based instrument vs NSW instruments.

  • mike kelly says:

    hi guys, i have been using distilled water because thats what my refractometer told me to use. has any body else done this? any input is helpful. mk

  • Joseph says:

    How does the accuracy (calibration aside) of a basic refractometer compare to that of a hydrometer or digital refractomer like the ones from Hanna Instuments for example? Do the digital refractometers need calibration as well? Is there any other method to find the specific gravity of tank water, since it is modified with salt, could a conductivity test be done with some type of conversion factor to determine SG?

  • Richard says:

    I’ll have to try calibration fluid. My refractometer readings rise in in the summer and drops in the winter. I end up using an Instant Ocean Hydrometer (because it is fast and easy) to assure the new water going into the tank has the same spg as the tank.
    I know hydrometer’s can give “false readings” but they seem to be very consistent. If my tank water reads 1.018 spg on the hydrometer (but is actually 1.025) and the water I am going to add to it also reads 1.018 spg, and both water sources are at the same temperature, then it is OK to do a water change.
    I sometimes “double check” using my refractometer, calibrating off either of the two water sources, and then testing the other. They always match.
    Every few months I take a water sample to my LFS to verify my spg is still good.

  • david t says:

    g’day, i have been using distilled water because thats what my refractometer operation manual said to use as well to reset to 0 at ambient working temperature of the room must be 20 c [68 f] whenever the instrument is recalibrated, if you dont have same distilled water on hand when you make your next coffe keep same of the balled water to use at room temperatuer

  • Joseph…the digital refractometers are likely more accurate. I’m not familiar with their product so I can’t say if it needs calibration as well. If nothing else, I’d recommend testing the refractometer against a known standard solution to check its accuracy.

  • Jhunsbar…I’m not sure what a “manual tester” is but I’m going to guess you are talking about hydrometers. Hydrometers are known to be widely inaccurate due to lots of factors including air bubbles on the swing arm, etc. I’d rather trust a tool I can calibrate instead of trusting something I can’t test.

  • Tony Viviano says:

    Thank you for this tip i did not know. I was told to calibrate with RO/DI water. wow!! I will get some Calibrating fluid ASAP. Do to the fact i want to go Reef tank soon. You have the best tips on the net thanks Mark.

  • Tony Viviano says:

    I am a tank dabbler now, but soon to be a Reef Enthusiast. should my salinity be at 1.025to start with? now for Fsih only I am at 1.022 PH-8.2 AMO “0” Nitrite “0” Nitate “0” Calcium 480+/- Thanks Mark!

  • Tony..fish only tanks can run @ 1.019-1.020.

  • Ray says:

    Another thing to consider too: if you are using a swing-arm hydrometer, try to switch to a refractometer. Calibrating a hydrometer is very tricky.

    Also, thanks Mark for posting my suggestion for the Friday a.m. Quick Tip!

  • Tony Viviano says:

    I have a good refractometer and i also have calibrating fluid as well, sorry i did not say that in my earlyer message. Thanks again for the tip. My quastion is should I bring my salinity up from 1.020/.022 to 1.025 if i am going reef tank, and what do you think of Red sea coral pro salt mix vs IO Reef Crystals. I have bin using IO salt mix for 1.5 years and now i switched to IO Reef Crystals. Should i stay or should i go Red Sea Pro.
    Thanks Tony V

  • Ray…I didn’t think a swing arm hydrometer was able to be calibrated.

  • Ross says:

    I think that was the reason it’s tricky Mark.
    Back when I had a swing arm type I used to calibrate it with NSW and a marker pen. It’s not a bad way to go about things when you are only just starting out.
    The biggest thing to watch out for after bubbles is the temperature of the sample. A few degrees will throw out the density of the sample by a fair bit.

  • Steve says:

    DO NOT use RO/DI to calibrate.. Use calibration solution.

    You should do some research on this Mark and revise your statement. If still unclear, contact RHF or Disc1 on RC.

  • Interesting that you are insisting on not using RODI when the instructions that came with the refractometer clearly state to use RODI water.

  • Matt says:

    Steve… The top makers of refractometers instruct you to use RO/DI at 000TDS…. I have three different brands and they all state the same…

    At the end of the day its up to you what you use but I don’t see any reason for a revision in Mark’s statement… His information is spot on as directed by suppliers of the equipment… It doesn’t get better than that!

  • Joseph says:

    Before I saw this post, I didn’t know much about any of this stuff. Since, I have done some research and reading and this is my take on what I’ve been able to gather.

    Some say calibrate with RODI, some say with calibration solution. I’ve not used one yet but I’m assuming that the calibration process involves putting the solution of know specific gravity on the frac and adjusting until it displays that reading. Yes, no?

    Some fracs are designed to measure NSW (natural sea water, I didn’t know what that stood for), from what I’ve read, most are for salt water. While the refractive index (what is actually being measured) is similar for the two, it is not identical. So, the scales for each would be slightly different. Again, assuming based on what I’ve been reading.

    For these reasons, I think it would be best to calibrate with a solution that is closest to the goal measurement. If your frac is designed for salt water and you are measuring aquarium water (which I believe would be more like NSW?) and you calibrate with RODI to 0 then your reading could be off by a factor that is multiplied over the distance from 0 to say 1.025. If your frac reads 1.026 with calibration fluid for 1.026 then the margin of error when reading tank water at 1.025 will be greatly reduced.

    Another way to think about it, -40 degrees on the Celsius and Fahrenheit scales are exactly the same physical temperature. The warmer you get, the bigger the discrepancy between the value of the two scales. If you calibrated your thermometer with a sample that is -40 degrees, it would be right on whether it was Celsius or Fahrenheit… at -40 degrees. If you thought it was a Fahrenheit thermometer, you would be perplexed when you measured water that was just about to boil and it only read 98 degrees instead of almost 212 degrees. If your goal was to measure when water was about to boil and you didn’t know if your thermometer was F or C, you would be better to calibrate your thermometer with boiling water to 212 (or 100 for that matter) degrees than you would to -32 (or 0) with ice.

    All of this reasoning mainly applies when using an NaCl frac for tank water. If you were actually measuring pure salt water or your tank frac was designed for NSW then I can see why it might be best to follow makers’ directions to use RODI water to calibrate to zero as that could be a more consistent control than calibration fluid which might vary from contamination or evaporation.

    All IMHO as I have no practical experience with any of this yet 😉

  • amanda says:

    do your quarantine your fish be for you introduce them into your tank?

  • Beetle Bailey says:

    Another great tip from Mark ,to be honest I’ve never even heard of calibrateing fluid , I will have to see who sells it over here in OZ,hope my salinity is ok !

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.