Don't waste your time on a Hydrometer, if you care for accuracy. (IN THE LONG RUN)
** IF THE AMAZON ONE is ALL PLASTIC (But METAL FAKE LOOK). GARBAGE. I got one and was terribly disappointed. **
I've been in the hobby for over 25 years, and in the Early days, I used Hydrometers. I still have my original two (one small, and one larger).
However, I'm talking about the classic Hydrometer that you fill up with water and the arrow gives you a reading.
They may work for a while with some degree of accuracy, possibly little less accurate depending on the quality of the unit you get.
But in my experience they go bad after a while.
I learned that when my first one went bad, so I got the 2nd one.
Maybe that happened since I didn't properly clean it after use.
I figure it was build up on the floating needed, or something else.
BOTH my hydrometer are NOW off by 0.004 (ie my Water is 1.025) but by hydrometers read (1.021/1.022).
How do I know.
When about a couple months ago, I was lent a REAL (PERCISION) HYDROMETER.
https://www.amazon.com/Tropic-Marin-ATM51002-Precision-Hydrometer/dp/B0002DK1YU/ref=sr_1_sc_1?ie=UTF8&qid=1513140978&sr=8-1-spell&keywords=topic+maurin+hydrometer
This one is accurate to 0.0001, and I tell you it's accurate if you have your water set to 25 degree Celcius, since only a cup or two of of water makes the reading change in a 15 Gallon Garbage can.
Using that TM hydrometer I discovered that even my Refactometer (China Cheapo, all Plastic) was off.
I got the cheapo since my good old quality hydrometer got rusty at the Calibration screw and I couldn't calibrate it anymore. (SO ADVICE, keep the cap on, and don't get salt water on the screw)
But what really bother me was not the offset with the CHINA plastic unit, it was the fact that I got different readings each time, and a light tap would change the ready. SO basically GARBAGE.
I did a test comparing all of them, and bought a SYBON FG100a which was the most accurate, and always gave a reliable/consistent reading.
But do your own research since another feature is the viewing window range-reading print. Some are tiny (hard to read), other are Larger (Better to Read). The FG100a is in between.
BE careful not to get a Beer Hydrometer since some have a range not for your AQUARIUM NEEDS.
But the TM Percision Hydrometer is dead on.
Not for everyday use. You need a deep pail and you need to get your water to 25 degrees exactly for that 0.0001 accuracy.
And water need to be dead still, while it bobs up and down for 5 minutes, and then settles to give reading.
After getting things right with the Precision Hydrometer....
I made a few bottles of exact Salinity SOlution (1.0250) and now I calibrate my Sybon Refactometer with that.
I also learned that there are two ways to calibrate a Refractometer (based on instructions provided).
1) Calibrating with Pure 0.00 Water (Is off quite a bit. Don't recommend)
2) Calibrating with standard 35 ppm solution (Very accurate, but not dead on)
3) And what I now use calibrating with a solution 1.0250 (that I know is what I want my tank to be) ((* HIGHEST ACCURACY* for refractomer ))
AND LAST BUT NOT LEAST (HOW TO USE A REFRACTORMER PROPERLY):
This is only if you want good reading/need accurate readings.
Since I had made dead-on accurate calibration solution 1.0250 I tested my trusty Sybon many times calibrating it to 1.025 (right in the middle of the line).
TIPS:
0) Rinse your hydrometer sample window with clean water. Wipe dry with paper towel.
1) Rinse your Sample-syringe a few times with RO or TAP water
2) Then rinse your Sample with your tank or made water.
3) Fill the whole area of the Glass hydrometer with a bead of water.
4) NO bubbles
5) The drop the clear flap over the bead (Press down LIGHTLY if needed, but not recommended, redo the flap drop to get a good SPLAT) FLAP will drop nicely only if your hinge is loose, not stiff.
6) There should again be no bubbles or air patches
7) Wait for a while for the temperature of the sample to match the temperature of the refractometer window (1 minute at least)
8) Then look into the viewfinder and get your reading in bright light (Read the same way each time. On blue line, above, below, whatever your fancy)
9) If you can't see the clear blue reading line (Tilt the hydrometer upward, and the line will get DARKER)
10) Of course adjust the focus too if your hydrometer has one (most do)
11) Rinse and dry your hydrometer clean after use
MAINTENANCE:
12) ** IF YOU remove the calibration screw cap for calibration, PUT IT BACK ON and never get screw wet, or else it will rust (Like mine did over the years)
13) ** that hinge on the clear flap is metal and it will rust prevent easy movement, if you wet it rinse it and dry it
14) ** Re-CALIBRATE each time you use it (A small bottle of calibration solution will last your years)
The SYBON I got cost more, but to me was worth EVERY Penny, and cost me less TOTAL COST than all the garbage I bought over the years.
The weird part is the Cheapo China (all plastic Metal looking fake) has the
BEST VIEWER PRINT (Large, wide lines, easy to read) but
all looks, no works.
I got this one.
https://www.aliexpress.com/item/Salinity-Refractometer-Meter-Water-Reader-Marine-0-10-Salt-Aquarium-Test-Tester/32741180326.html?spm=a2g0s.9042311.0.0.5NGy8d
If you pick it up in a store and compared it to a quality metal refractometer, you'd probably buy it. So that's their sell point (That viewer display).
All the best in your selection.
Hope this reply helps. I actually stumbled on your post by accident. And recently did my analysis/testing so had this all fresh in my head (Photo from a post I did on my Tank Thread)
