Refractometers and The Importance of Measuring Salinity

Welcome to Simple Reefs. We are a brand new website designed to take the stress and expense out of starting your very own marine aquarium. We’ve been talking about marine aquarium test kits a lot, lately. One really underrated aspect of aquarium testing that a lot of hobbyists neglect is the importance of measuring salinity using a refractometer or hydrometer.

Today, we are going to talk about why it is vital to establish a routine of measuring your salinity and how salinity can impact your tank’s residents. We are also going to talk about some of the devices available to the marine aquarium hobbyists to help them effectively measure their salinity.

The Importance of Measuring Salinity to a Marine Aquarium

Salinity is one of those hidden numbers that is actually incredibly important to the health of your marine aquarium. In fact, you could make the argument that it is the most important parameter in your entire marine aquarium. The importance of measuring your salinity is impossible to understate.

But why is that? Well, the reason is quite simple and fully understanding it is really quite eye opening. The salinity of your aquarium directly impacts the osmotic balance of every single living creature in your tank. Even those annoying bristle worms that come crawling out at night.

So what is Osmotic Balance, you may ask? Well, osmosis is the movement of water over a semi-permeable membrane, in this case, a cell membrane (because we are referring to animals not plants). The osmotic balance is the balance of solute concentrations (like salt) in the cell compared to the concentrations in the surrounding environment.

In a marine environment, the concentration of salt in the water is higher than that of the salt in the fish’s body. This causes something known as Osmotic Pressure. In essence, this means that the salt in the water is continually attempting to pull the water out of the fish’s cells and body. This would obviously be incredibly bad as it would dehydrate the fish and its cells.

An equilibrium needs to be maintained between the solutes in the cell and the solutes in the surrounding environment. This balance allows the proper passage of water into and out of the cell. Without this proper balance, the cell could receive too much or too little water causing it to shrink or inflate which will damage its structure.

A Delicate and Crucial Balance

This is where evolution has stepped in. The residents of your tank are constantly regulating their internal salinity in a process known as Osmoregulation to manage this delicate balance. Osmoregulation is a pretty incredible adaptation to the harsher conditions of the sea and saltwater.

The fish in your marine tank are constantly taking in lots of seawater and then, using specialised cells, excrete the excess salt through their gills. Salt water fish only excrete small amounts of highly concentrated urine and their bodies are adapted to conserve water.

In ideal conditions, and with correct salinity, this osmoregulation is not an issue. Sure, it is energy intensive but the creatures have adapted to do this as part of their day to day existence. After all, those are the conditions they have evolved to thrive in.

When your salinity is too high or too low, however, conditions suddenly become far less than ideal. The creatures in your tank will have to work harder to maintain this osmotic balance. When any organic being is forced to work harder just to maintain normal living conditions, there are consequences. They will have a higher metabolic rate which will burn additional energy and stress will be the result.

This stress will cause the creature’s immune system to weaken which will, in turn, eventually lead to premature death. When you think about the importance of salinity in a marine aquarium in this context, it stops being just a simple number. It suddenly feels a lot more like the crucial and important life support system that it is. The salinity of your aquarium is vital to the health and wellbeing of your fish and coral.

Salinity is a Frequently Neglected Parameter

Salinity might be the easiest to test parameter in your entire aquarium but also might be the most neglected. Salinity drift can happen for a number of reasons that we will go into later this week. As I mentioned in my water testing schedule article, you should really be testing salinity once a week.

It’s actually somewhat concerning to see the amount of people, particularly beginners, who don’t even have a way to measure salinity. Stop me if you have heard this one before but it is something I see all the time.

Someone turns up on an online forum or Reddit asking for advice. People wanting to help ask them for their parameters including salinity. They come back asking the person helping how they measure their salinity.

It is paramount to not get into the saltwater fishkeeping hobby without a way to measure salinity. It was the first thing on my list that I was sure to check off. Simply following the instructions on your salt mix isn’t enough to guarantee accuracy.

The salinity that they claim your water should be after mixing isn’t always the actual salinity of the water you mix. There are numerous factors that go into it and results can be different almost every time. Humans are very prone to errors, particularly in tasks that they do infrequently, so mixing salt this way isn’t a perfect science.

Salinity Can Be a Perfect Science

Well, salinity isn’t a perfect science until you have a calibrated scientific instrument to make it as close to perfect as possible. That’s where tools to measure salinity come in and I have some really good news for you. An extremely precise and easy to use refractometer is incredibly affordable. In fact, you can pick one up for less than £15 on places like Amazon.

You may be wondering why I have specifically mentioned a refractometer to measure salinity and not one of the other tools available like a Hanna digital salinity checker or a hydrometer? There are a couple of reasons.

• Hydrometers are notoriously unreliable: Hydrometers are the “old fashioned” way to measure salinity and they are still around today. In fact, you can grab a Fluval branded one for less than £8 on Amazon. The problem is, their accuracy is impacted by a number of different factors. Air bubbles can cling to the swing arm impacting the result. Salt creep can build up on the internal mechanism. They can be incorrectly calibrated right out of the factory and temperature really throws off the reading. I have known these things to be out by a multiple ppt (0.01 SG – 0.02 SG). That’s a massive problem when you are aiming for a specific salinity.
• Digital Meters are Expensive: Simple Reefs was designed with the intention of saving reefers money. With that in mind, it is tough to recommend a reliable digital conductivity meter (like a Hanna salinity checker, for example) when they cost in excess of £70. These meters are very precise but you need to be very sure that they are calibrated well following the manufacturer’s instructions. They will take a big chunk out of your wallet, too.

If you don’t already have a way to measure salinity, you need to change that as soon as you have finished reading this. The health of your tank’s residents depends on it.

Refractometers – An Affordable Way to Test Salinity

Refractometers are one of those pleasant surprises when it comes to the marine aquarium hobby. They are effective and extremely affordable, but how do they work? Well, it’s quite simple and all starts with light refraction.

Light refraction is the way in which light bends as it passes from a medium with one density to a medium with another density. It sounds a bit complicated but there are examples of light refraction all around us. Take the strange way a road on a very hot day can look almost like shimmering water. This is because of the way light bends at it travels through the layers of air coming off the road with different temperatures and densities.

A refractometer works by measuring the angle at which light bends when it passes from the air, through a water sample, and into a high-density prism inside the device. You simply take a drop of the water that you want to test, place it on the test plate, and look through the magnified result lens.

In an upcoming article, we are going to talk all about how salinity is measured. The main thing we need to know here is the fact that water containing salt is going to have higher density than freshwater. This means saltwater will bend light more than freshwater.

The saltier the water, the denser it is, and the more it will bend light. This angle of refraction, known as the Refractive Index (RI), is directly proportional to the water’s salinity. This means it can be effectively measured and translated into a number that we can use to measure salinity – Specific Gravity (SG).

Refractometers – A Simple Yet Effective Design

The vast majority of basic, analogue, refractometers available to hobbyists are going to consist of the same bits and pieces.

The Daylight Plate: A hinged and transparent flap at the end of the device. Lifting this reveals the prism.

The Prism: A small, flat, blue-tinted, glass surface where you place the water sample. This is where the light first enters the device. You are going to want to keep this very clean, refractometers usually come with a cloth to wipe it down. Make sure you dry it off after use because they are prone to damage from salt creep.

The Scale: An internal, calibrated reticule that becomes visible when you look through the eyepiece. In aquarium models, this scale is often marked in both Specific Gravity (SG) and Parts Per Thousand (ppt) of salinity.

The Eyepiece: This is what you look through to view the reading. The reading will display as a line corresponding to the level of salinity of the sample. You will also find a focus dial that you can twist to focus in on the result, sort of like the lens on a camera.

Automatic Temperature Compensation (ATC): Since water density changes with temperature, Automatic Temperature Compensation employs a small bi-metallic strip that expands or contracts, subtly adjusting the optics to correct for temperature variations within a specific range (usually 10-30°C or 50-86°F). Room temp can vary quite a lot so this ensures your reading remains accurate even if the room temperature fluctuates.

The design is incredibly simple but works tremendously well. The simple design means a refractometer is extremely affordable, too. You can pay a little extra for branded examples but the differences are minor.

I would suggest making sure you purchase a marine aquarium refractometer as they will be calibrated for the task at hand. Other refractometers may give incorrect results as they are calibrated for various different fluids.

How To Use a Refractometer

Using a refractometer is a straightforward process. With a little practice, you will be able to test your water in a few minutes. I would point out that it is handy to find a spot in your house with strong outdoor lighting as that can make reading the device much easier.

Step 1: Calibration (Very Important!): Before its first use, a refractometer must be calibrated to ensure accuracy. This is done using pure Reverse Osmosis Deionized (RO/DI) water. Place a few drops of RO/DI water on the clean prism, close the daylight plate, and look through the eyepiece. The reading should be exactly zero (1.000 Specific Gravity).

Don’t worry if it isn’t, refractometers come with an adjustable calibration screw. Use a small screwdriver (sometimes this will be provided) to turn the calibration screw until the line between the blue and white sections sits perfectly on the zero mark. Your refractometer is good to go!

Step 2: Clean The Refractometer: Clean the prism with a soft cloth. The kits I have include a very small cloth for this purpose. Get rid of any smears and smudges.

Step 3: Take a Sample: Using a pipette (Again, this will probably be included), collect a sample of your aquarium water from an area with good flow. Keep away from any freshwater top-off points as this could corrupt your result. Place 2-3 drops of the tank water onto the blue prism part of your refractometer.

Step 4: Close the Plate: Gently close the daylight plate over the water sample. Don’t let it slam or you are going to lose a lot of the water sample. Some of it will probably end up in your mouth or eye, as well, because of Sod’s law. The water should spread evenly across the prism’s surface without any large air bubbles.

Step 5: Reading the Scale: Hold the refractometer up to a natural light source or a bright room light and look through the eyepiece. You will see a clear line where the upper blue field and lower white field meet. The point where this line intersects the scale indicates your water’s salinity.

Step 6: Clean and Dry The Plate: Don’t just put your refractometer away. Open the flap, shake off the water, dry it gently and then give the plate a wipe with the small cloth. You can even put a drop of deionised water on the end and wipe it completely clean. Refractometers can be damaged by the salt in the water so this step will prevent that from happening.

What Should My Marine Aquarium Salinity Be?

This is a subject we are covering thoroughly in an upcoming article. Suffice to say, it depends on your setup. If you have fish only, you can opt for a salinity between 28-33 ppt (SG 1.020-1.024). If you have a reef tank with invertebrates and coral, you will want to go a little higher to around 35PPT (SG 1.026).

This is a bit of a loaded subject and we offer a few different options depending on your situation in the article I just mentioned. Fish only tanks can benefit from lower salinity. The ideal, when it comes to reefs, is to match as close to the actual ocean as possible. After all, this is what these creatures are adapted to.

In Short

To summarise, proper salinity is vital to the long term health of your aquarium’s residents. Adequate salinity helps the creatures that inhabit your tank to regulate the salinity inside their bodies. A cheap and accurate device like a refractometer can be used weekly to measure salinity and help you achieve a consistent and stable level. Thanks for reading and spending your time with Simple Reefs.