May have to invest in a refractometer

They're both accurate. Just make sure you wash a hydrometer and there are no bubbles on the swing arm.

 

Get a Refracto with 35PPT solution, your entire tank will thank you with growth and colors.

 

I respectfully disagree, Mel. Statements like that insinuate that SW aquaria would be impossible without one, which is simply not the case. In fact, the hobby was quite successful several decades before the refractometer was created, and even more so after the advent of RO/DI filtration. Furthermore, the refractometer is not infallible; it's accuracy depends on proper care and maintenance, same as with anything else. I will agree that swing-arm hydrometers, by design, can be inherently inaccurate, but there are ways to improvise, such as my original suggestion.

All that said, I've never owned a swing-arm hydrometer or refractometer, nor will I. The floating glass hydrometer, which was all we had in the early days, has served me well for 39 years now, with 100% accuracy. If it's good enough for laboratories, then it's good enough for me.

 

My hydrometer works great. Make sure you rinse it out with fresh water and tap the bubbles off the arm. My friend spent over 80 dollars on his refractometer and the few times we compared it was almost spot on.

 

Just to add,

I'd get at least one more test of your hydrometer. How do you know the lfs's is accurate?

FWIW, I rinse my Hydrometer in RO water- maybe that's what they meant by freeshwater?

I too have a refrac. now- If you have 0 TDS water, can't you use that to calibrate it to 0 also? I know they they no, but it seems to be working for me right now. I'll probalby break down and get so calibration fluid though.

Inherently, what's more accurate, a solution bought that is claimed to be 35ppm, or KNOWN 0 tds, that you know is 0? Makes me want to go Hhumm.

 

Todd~ What I've heard from most users is that the refractometer must be calibrated to the intended target value for maximum accuracy. IOW, if you calibrate to zero, you'll only be 100% accurate at or near zero. Like anything else though, I'm sure some models may be better than others.

 

Mr. Bill is spot on, if you calibrate to zero you can be 4 or 5 units off.

 

Todd, I thought you learned your lesson and got calibrating fluid, lol! Go to Best Fish and get some!!!

Hydrometer or refractometer, I would still want to compare them occasionally to a known standard. I wish more test kits came with one to test against, I would be more confident in the results. A standard comes with the Seachem copper test kit. It really helps when dosing Cupramine to be sure you don't, you know, kill all your fish.

 

In my opinion...

The swing arm hydrometer is great and all you need in a fish only system.

I think the refractometer is a necessary tool for the reef.
(Or as MrBill says, a quality floating glass hydrometer. Many of the reefing books refer to this style as well)

And NO, you can't calibrate a refractometer with rodi water. You will NOT get accurate results.
I picked up my refractometer locally and didn't have solution for a while, but thought I was fine calibrating with rodi. When I finally picked up calibrating solution, I realized it was waaaay off!

I calibrate before EVERY test now. Because sometimes its outta wack, sometimes its spot on. I can't explain why but it needs to always be verified.

 

Temperature I calibrate every time as well.