Category Archives: Equipment

Tips for Calibrating Your Aquarium Refractometer

Few things can get your new aquarium off to a bad start quicker than a refractometer that isn’t properly calibrated. After all, saltwater is the first thing that goes into your new tank – get it wrong and you’re doomed to problems right from the start.

I almost learned that the hard way…

A used refractometer was posted for sale on my local Marine Aquarist Society forum, so I grabbed it.  It was in fantastic shape and I figured I could use the money I saved for some other equipment. The person selling it even tossed in a bottle of refractometer calibration solution.

The instructions that came with it called for using distilled water and calibrating it to zero. However, many aquarists believe it is better to use calibration solution that has the same salinity as natural saltwater (35 ppt, or 1.0265 specific gravity at 77° F / 25° C) since that is the specific solution/liquid they’ll always be using it to test.

To get ready for my first batch of saltwater, I let both the refractometer and the bottle of calibration solution sit in a room that was as close to 77° as possible for a few hours. Then I placed a few drops of solution on the refractometer and calibrated it to 35 ppt.

Then I mixed my saltwater.  It needed a little more salt (I’m using E.S.V.’s 4-part saltwater mix) than the instructions called for in order to reach 35 ppt. Other than that it was smooth sailing – the refractometer worked great. However, it turns out the calibration solution did not…

The day after mixing my saltwater and putting it in my new aquarium, the water turned hazy. At first I thought it was a little bacteria bloom – which is quite common in new tanks. But the next morning I noticed the water had cleared and left a white dust on the bottom of the tank. Something had precipitated out of the water – most likely calcium, possibly salt.

My immediate thought:  something isn’t right with the refractometer. So, I re-calibrated it – this time following the refractometer instructions and using distilled water (actually I used RODI water.)

Sure enough, the original calibration had set the meter way too low. Then I realized my mistake – I never asked the previous owner how old the bottle of calibration fluid was or how it was stored, and there is no expiration date stamped on the bottle. There was a VERY good chance some of the base liquid evaporated off, or the solution has surpassed its shelf life.

Thanks to a very handy online article Randy Holmes-Farley wrote on homemade salinity calibration solution in Reefkeeping Magazine, confirming my suspicion was pretty quick and easy – all it took was a quick trip to the grocery store for a 79 cent tub of table salt (no iodine added).

Per Mr. Holmes-Farley all you need to do is dissolve 36.5 grams of table salt in 963.5 grams (e.g. mL) of distilled water (again, I used RODI water). And this is a scale-able ratio. So, using a kitchen scale to measure, I just mixed 18g of salt in 481.75 g of RODI water. (The article also gives directions for using measuring cups if you don’t have a kitchen scale.)

I used my fresh, homemade solution to calibrate the refractomer. And then took a reading of RODI water on the refractometer – which measured exactly zero, confirming the refractometer was now properly calibrated.

With the properly calibrated meter in hand, I checked the water in my tank next.

Sure enough, the salinity was way too high.

Fortunately there was nothing in my tank yet but the saltwater. So I simply replaced a half-gallon at a time with some pure RODI water until it came down to 35 ppt (or 1.0256 specific gravity).

If this had happened when I had livestock in the tank, the results could have been disastrous.

So, here are three quick tips for calibrating your refractometer:

  1. If you use a manufactured calibration solution – whether it’s store bought or second-hand, make sure it’s relatively new and was well stored
  2. Better yet, make your own solution – it very easy, you KNOW it’s fresh each time you use it, and it costs less than $1 and 5 minutes to make
  3. Make sure your refractometer – not the calibration solution – is as close the calibration temperature stated in instructions that came with it as you can. (Don’t worry about the calibration solution temperature:  you’re only putting 2-3 drops of solution on the refractometer prism – within 30-45 seconds it will be the same temperature as the refractometer.).



RODI Systems — Do You Need One?

Most experienced aquarists consider this THE most crucial piece of equipment for a saltwater aquarium — and yet, it almost always hidden in the background… no one ever sees it… and many people just starting out in the hobby have never even heard of it…

What is this magical — and often expensive — piece of equipment?  It’s a Reverse Osmosis Deionization system, or RODI system. Or, put simply, a water filtration system.

Essentially, water flows through one to three filters to remove chlorine and larger mineral and sediment particles, then through a reverse-osmosis membrane to purify it further, and finally through a deionization cartridge to remove the few, final impurities that were left.

I am by no means a biologist and have no intention of trying to boring you with any more details on how RODI systems work. If your curious you can find a ton of info on forums like

The big question is:  Do you need an RODI system for your saltwater aquarium?

It depends…

It goes without saying that reef aquariums are very sensitive and complex ecosystems — even very small traces of chemical (or mineral) impurities can quickly trigger very nasty side effects and cause a great deal of harm. When adding fresh top-off water to your tank, or mixing new saltwater to do your water changes, it is very important to use water that is close to pure H2O as you can get.

With that said, you have three choices:

  1. Use your tap water and take a BIG risk of poisoning your system (The chlorine/chloramine alone in tap water, and the heavy amount of minerals in most well water, can cause significant harm to the inhabitants of your reef tank)
  2. Make frequent trips to the local pet store to purchase jugs of RODI water. The larger your aquarium, the more jugs of RODI water you’ll have to regularly haul home
  3. Get an RODI system which, while initially expensive, can quickly pay for itself several times over in the form of time it saves you, livestock it keeps healthy, and gas and money saved on trips to the pet store

What kind of RODI system is right for you and where can you get one?

RODI systemRODI systems are based on the amount of purified water they’re capable of producing in one day. For example, under typical conditions a 100GPD system will produce 100 gallons of pure water every 24 hours.

The size of your aquarium will determine the what size RODI system you will need.

And your water source will determine what type of filters it will contain.

If you’re on a city water supply, you’ll need to ask your water company whether they use chlorine or chloramine. A RODI chlorine cartridge will not remove chlorine, so you need to choose an RODI system with the right type of cartridge.

It may also be worthwhile to ask them what the Total Dissolved Solids (TDS) of your tap water are. They do regular testing of their water as a matter of public safety and will share the most recent test result with you if you ask.

If you are on well water, you may want to consider a cistern or container to some well water in for a few days before running it through your RODI system. The higher levels of CO2 in well water can wear out the reverse-osmosis membrane much quicker than usual and it is the most expensive cartridge to replace on a RODI system. Allowing the well water to sit for few days before filtering it will allow some of the CO2 to naturally degas out into the air before you filter it.

Whichever RODI system you choose…

Make sure it comes with a TDS meter!

It’s important to measure each batch of finished water to ensure is has a TDS of zero (or at the very least less than 4, although it’s best to have 0 TDS). When your TDS starts to rise above that, you’ll know it’s time to replace one or more of the filters.  Any respectable RODI system will, at the very least come with a hand-held TDS meter so you can test your finished water. Many good RODI units come with one or more TDS meters built into them (so you’ll know which specific cartridge needs to be replaced).

I realize this all may seem a bit confusing at first, but once you look at a few it starts to become a bit more simple…

I suggest starting your search on websites the of Spectrapure, Buckeye Hydro, and Air, Water, and Ice — all of whom are makers and sellers of highly respected RODI systems. Then, check out online saltwater aquarium stores such as and and read reviews of some of the RODI systems they sell.