Based on all of the reading i have done it seems like you will get the most accurate readng using this calibration fluid rather then RODI. I know my measurements appear to be more accurate after calibrations with this fluid.

Great product to use, makes calibrating very fast and easy.

With the amount of money I have in my tank there is no way I would not have a refractometer. That said, without an accurate calibration fluid the refractometer is useless.

This fliud does the job and does it well. This is the second bottle I have re-ordered from BRS and when it runs out, I'll be ordering it again.

Marine tanks are too costly to go cheap on the salinity, so having a properly calibrated refractometer is essential. This does the job perfectly, at a very reasonable price. So many people can't be wrong about this... use the calibration fluid!

turns out my calibrating with ro/di had my refractometer off by .002
Calibration fluid was well worth it

This solution is the back bone to keeping my paramount check. 4 stars because I do believe it's a little pricey. Having said that it is a decent sized bottle and the built in dropper is great. I will not have to buy this for a while so that makes up for a dollar or two difference I've seen in lesser quality solutions.

This is a great item to keep on hand. Having a refractometer not calibrated correctly can cause major issues. There is enough fluid to last for a very long time. Great product!

ok folks, add this to the "must have" list of your tank maintainence and testing supplies! I have always calibrated to 0ppt using ro/di. after getting this calibration fluid I discovered by refractometer was off by 2.5! so, i was actually mixing my salt to 1.023 1/2 instead of 1.026.

I was using RO/DI water to calibrate and I am glad I switched to this! My refractometer was off by quite a bit! The built in dropper is nice as well!

was going to make some DIY calibration fluid but felt ALOT safer getting this . even if you are going to do some DIY fluid is it a good idea to double check it with this.
product is very easy to use glad I got it.

Great product, I found that my readings were way off until I started using this product. ThankYou BRS!

My salinity was off by about 1ppm. This came in handy.

This is a great product. You sould always calibrate to 35ppm. Never calibrate with tap water or even ro/di.

What a great product.
No need to run around looking for a pipette which could anyway be covered in salt creep :)
Just drop it on and calibrate. So easy you will find yourself calibrating before every water change :)

Anyone who owns a refractometer should buy this product. Having a fixed salinity to calibrate your equipment to is a no brainer. Stop relying on RO/DI water to calibrate your refractometer, you could be off and not even know it. I feel more comfortable now knowing that my equipment is correctly calibrated.

I tried using distilled (RO/DI) water, and was getting terribly inaccurate results. I didn't know that you actually can't get a good reading after calibrating with distilled water. After buying this calibration fluid, my readings are spot on, every time. This is a great product. Highly recommended.

Although they say you can calibrate a refractometer with distilled water, I would rather trust something that I know exactly what the SG is. This is something I would never go without. Must have to ensure proper calibration of your refractometer

Easy to use and get perfect readings for your gravity

If you have a refractometer, this is a must like the other review states. Quick and easy calibration to 35 ppt. Doesn't drip and isn't messy at all. I use to calibrate with RO water to 0 ppt and after using this I realized I was still slightly off. It makes a difference being able to calibrate against something close to natural salt water.

I actually talked to Michael Del Prete (Aquacraft CEO and Inventor) about this and this is what he had to say:

Testing salt water for the proper specific gravity or salinity is the first, easiest and most important part of all successful marine fish and reef aquariums.

Accurate calibration of a refractometer is essential.

The vast majority of refractometers do not read accurately when calibrated at -0- with RO, RO-DI or distilled water and salt water with a SG 1.026 / 35 ppt. salinity is tested.

A miscalibrated refractometer of +/- .002 SG units or +/- 2-3 ppt. salinity units will yield an inaccurate test result for calcium of +/- 75-100 ppm.

All other ions will be +/- according to their amounts present in the salt water solution.

Simply calibrate the refractometer at 35 ppt. salinity or 1.026 specific gravity using this precisely formulated scientific standard calibration solution.

Aqua Craft Products® Refractometer Calibration Solution has the same refractive index of 1.3394 @ 77F / 25C as natural sea water.

Learn your exact salt concentration and get accurate and reliable test results. Keep your captive marine pets in optimal living conditions.

This is the ONLY solution made specifically for calibrating refractometers for use in salt water.

Aqua Craft Products® Refractometer Calibration Solution is a must.

2 ounce / 60 ml size. With daily use of RCS, this will last about one year.

Dropper top. Simply put a few drops on the refractometer lens. No spilling. No wasted solution. No mess.

Instructions and contents are clearly labeled.

Like all Aquacraft products, this is made in the USA using proprietary methods and strict quality measures.

I have been using this for about a year now and still have some left. A little goes a long way. This is a "must have" if you use a refractometer.