Still using a creaky old swing-arm box-type hydrometer? Or a delicate glass hydrometer that shatters at a moment's notice?
You know the score: Constant washes in vinegar to keep them working properly: fill to the top / line, poke with stick, water goes everywhere... hope for the best.
Here's me measuring the salinity of my GSP tank... 1.021. Or thereabouts, thanks to the helpfully blunt pointer (NOT). A bit on the low side for my liking, as my GSPs are in full saltwater.
Look at the mess!
There is a better way: Refractometers. These things are just as they sound -- they use a glass prism to refract light. The angle of refraction depends on the salinity of the sample.
Look through the eyepiece to see scales of salinity and specific gravity.
(it is much clearer than this in real life - very hard to get a shot of the scale!)
Let's test the GSP tank again. Very simple to use: Put a few drops on the glass, and close the cover:
Look through the eyepiece:
1.026 - 1.027! Perfect!
Compare the reading: 0.006 difference!
that's a huge difference for any marine or brackish tank. Do you have a brackish tank where you are varying the salinity for F-8s or GSPs? Would you be shocked to know that your SG of 1.018 could really be 1.024? Or 1.012?
Now, refractometers, if not calibrated properly, can be as unreliable as hydrometers... but in general, if looked after properly, they are more accurate.... and, as you can see, they are much, much easier to use. IN addition, on auto-temperature compensation refractometers (which is what you're likely to find), temperature calibration is not an issue.
Looking for one more thing to buy for your tank? Look no longer! In my opinion, a refractometer is about the best investment you can make. Since I purchased this about a year ago, I have been much more confident about the salinity of my tanks, and I will never use a swing-arm hydrometer again. And it's a cheap investment, at well under $50.