Marine Aquariun Basics, Part 1: Water and Salt

Author: Philip Hunt

One of the biggest challenges freshwater hobbyists encounter when switching to saltwater systems is getting the water chemistry just right. An expert hobbyist reviews the best practices for mixing up the perfect batch of saltwater.

Getting Your Sea Legs

In this series of articles, we'll be going back to the basics of marine fishkeeping, providing the kind of information that anyone starting out in the hobby will find useful and that will also provide a reminder for more experienced enthusiasts. This month, we'll start with water and salt—the most basic elements of the marine aquarium. Together these make up the artificial sea water that most marine fishkeepers use to fill their aquariums. While some fishkeepers collect water from the ocean and others buy ready-mixed artificial sea water, mixing a commercial salt mix with fresh water is the norm, so that is the focus of this article.

Water: 96.5% of the Issue

In natural sea water, the total salt content is about 35 parts per thousand, or 3.5 percent, which means that 96.5 percent of sea water is water. From this simple statistic, it's easy to see just how important it is to have water of high quality for the aquarium. But what does high quality mean in this context? Surely the water in your domestic supply is high quality if it's good enough to drink, right?

Reference to natural sea water around coral reefs provides the answer: The water in such areas has extremely low levels of dissolved plant nutrients, such as nitrates and phosphates, and the inhabitants of coral reefs have evolved to live under such conditions. To some of these organisms—most importantly, from the aquarium perspective, corals and their relatives—these nutrients are deleterious. In addition, as might be expected, high levels of plant nutrients will encourage vigorous algae growth, especially in the brightly lit environment of a reef aquarium.

Nitrates and Phosphates

Domestic water supplies often contain significant levels of nitrates and phosphates. This is the result of a number of processes, the most important of which is probably agricultural use of fertilizers that eventually find their way into the water supply. At the levels usually seen, these don't appear to have adverse effects on human health. However, if your water supply does have significant levels of nitrates and phosphates, it's not a good idea to use it to make up artificial seawater: the result would be water with maybe 500 times more nitrate in it than your corals would like. Some method of purifying the water is needed to remove harmful dissolved substances so that when a salt mix is added, the result is something close to the water around a coral reef.

Cleaning the Water

Three main water-purification methods are used for aquariums: reverse osmosis, deionization, and ion exchange.

Reverse Osmosis

Reverse osmosis (R/O) systems work by forcing water under pressure through a membrane that acts as a kind of molecular filter: Small molecules, such as those of water, pass through the membrane, but larger molecules, including nitrate and phosphate ions, are held back. The process produces very pure water but is not particularly efficient in terms of water use—in small systems, 90 percent of the water entering the unit is discharged as waste, with only 10 percent emerging in purified form. Domestic R/O units typically produce somewhere between 20 and 150 gallons per day.


Deionizers work by binding charged particles (ions) onto a special resin. Deionizers generally consist of a column packed with resin and sometimes also include a carbon filter to pick up other dissolved substances that can pass through the deionizing resin. Deionizers produce high-quality water, with no waste, and are relatively inexpensive to buy. They tend to run quite slowly, although they have a higher flow rate than small R/O units do—aquarium models might deliver 10 to12 gallons per hour.

The major downside of deionizers is that the resin binds all ions going through it, which in hard water areas means that the resin, which is expensive, becomes saturated with ions (i.e, exhausted) quite quickly and will then need replacement. Many deionizing resins have indicators that change color when the resin is exhausted, so it is easy to see when replacement is needed.


Ion-exchange systems provide an alternative to R/O units and deionizers. As with a deionizer, the water passes over a resin column, but the ions in the water don’t simply bind to the resin. In ion-exchange resin, each positive ion that binds displaces a sodium ion from the resin, and each negative ion displaces a chloride ion. This means that the water coming out of the unit contains sodium chloride, so it isn't quite as pure as water from an R/O unit or a deionizer, but in the context of using the water to make up sea water (using a salt mix that is primarily sodium chloride, of course) it is adequate.

Ion-exchange units are inexpensive to buy and have higher flow rates than either R/O units or deionizers. They are also inexpensive to run because when the resin is exhausted, it can be regenerated by running a concentrated solution of common salt though it. This has the effect of displacing any ions already bound to the resin, leaving it ready to bind more.

While many fishkeepers purify their own water, it is also possible to buy R/O water from many aquarium dealers. On a per-gallon basis, this is not usually very expensive, but if you're going to fill anything other than a nano aquarium, the cost does mount. Perhaps more significantly, transporting large volumes of water can be problematic, as it requires food-grade containers and the weight soon mounts to back-threatening levels.

Do You Need to Purify Your Water?

Domestic water supplies vary tremendously in their nutrient levels, so before investing in water purification equipment, it is worth finding out whether it is needed. While you could go to the trouble and expense of sending your water to a lab for a full analysis, simply testing the water for nitrate will, in most cases, tell you everything you need to know.

The two big problems with water, as far as marine aquariums are concerned, are nitrates and phosphates, and as both usually derive from agricultural fertilizers, they tend to be found together. A nitrate test is usually simpler, less expensive, and more reliable than a phosphate test (you can even get dipstick tests), so unless your water is very unusual, it will give you a very quick answer to the question of whether or not to purify.

So what is the exact nitrate threshold at which you should clean up your water? An obvious answer is anything above zero, and if you are using the water for a reef system, this is probably a good rule to follow. The ideal in a reef aquarium is to keep nitrates as close to zero as possible, and although there are many ways to control nitrates in the aquarium, it's probably best to avoid introducing problems that you'll then need to solve. If you're planning a fish-only system, while the same zero-nitrate approach is best, things are less critical, and nitrate levels up to 5 mg/L shouldn't cause problems.


While you might need to put in some work to get the water right for your marine aquarium, the salt component is a whole lot easier to deal with. Essentially, any of the commercially available salt mixes will produce good-quality artificial sea water. There are minor variations between brands with respect to their speed and ease of dissolving and the precise levels of certain components.

Some brands, for example, are formulated specifically for reef aquariums, having higher levels of calcium, carbonates/bicarbonates, and sometimes magnesium, which promote good stony coral growth—although it’s hard to say whether the higher levels of these ingredients in the salt mix make much difference in the context of an aquarium with either a calcium reactor or regular calcium supplementation.

The Right Mix

In general, it's best to aim for an aquarium salinity similar that of natural seawater, namely around 35 grams per liter (or 35 parts per thousand), equivalent to a specific gravity of about 1.026 (at 20°C [68°F]). While it used to be common practice to run fish-only aquariums at a lower salinity than natural seawater, typically about 28 grams per liter (specific gravity 1.022), this seems to have fallen out of favor recently.

There are some specific applications for different salinities, for example in low-salinity quarantine or for disease treatment, but for everyday fishkeeping, it's best to keep things as close to natural as possible. It's also good to keep the salinity in the aquarium stable, topping up evaporation losses regularly and matching the salinity of your replacement water and existing tank water when doing partial water changes.

Measuring Salinity

There are three main ways to measure salinity: using a hydrometer, a refractometer, or an electronic salinity meter.


Hydrometers measure the density of liquids, which in seawater is a function of three things, how much dissolved salt is present, temperature, and pressure. The density of the liquid is expressed as specific gravity, which in the case of seawater is the ratio of the density of the water being measured to the density of pure water. Pure water has a specific gravity of 1; therefore, natural sea water, with a specific gravity of 1.027, is 1.027 times denser than pure water.

While fishkeepers can basically disregard the effects of pressure on specific gravity, it's important to pay attention to temperature. Hydrometers are calibrated to work at a certain temperature, which is often 20°C (68°F). With some hydrometers (float hydrometers), it's necessary to use a table to correct readings taken at different temperatures. Others (swing-needle hydrometers), because of the materials they are made of, claim to be self-correcting for temperature. Hydrometers are an inexpensive way to measure salinity but are not always as accurate as other devices.


Refractometers measure the refractive index of liquids, which varies with the concentration of dissolved salts. Aquarium refractometers often self-correct for temperature. Usually more accurate than hydrometers, they are also more expensive, but not prohibitively so. They do require occasional calibration with pure water, which is used to set their zero point, and must be cleaned carefully after use.

Electronic Salinity Meters

Electronic salinity meters measure the conductivity of salt water, exploiting the fact that more concentrated solutions conduct electricity more easily. The meters then translate this into the equivalent salinity. Like refractometers, salinity meters need regular calibration (in this case using a standard salt solution) and careful cleaning after use. They provide an accurate method of measuring salinity but are expensive compared to both hydrometers and refractometers.

Which device you use to measure salinity is largely down to personal preference. Refractometers and electronic meters may be more accurate, but this comes at a price in terms of both cost and a need for calibration and more careful cleaning than hydrometers.

It is also worth considering that keeping your aquarium at a consistent salinity is more important than the exact value in parts per thousand, so provided your hydrometer is not wildly inaccurate, and you use the same one each time, you might not need a more sophisticated instrument. If, however, you will need to adjust salinity and do so very carefully, as, for example, when using reduced-salinity water to treat fish diseases in reef aquariums, then a refractometer or salinity meter will be a better bet.

How to Make Artificial Sea Water

There are two primary reasons for making up artificial sea water: filling a new system and preparing water for partial water changes. In most cases, when setting up a new system, the most practical way is to make up the water in the aquarium itself due to the volume of water required.

Fill the Tank

To do this, fill up the tank with water (purified if necessary), allowing some space for displacement by live rock, sand, or other substrates. If you are using a rectangular tank, you can measure the length and front-to-back depth of the tank, then the depth of water, and use this to work out the volume of water you have added. For other tank shapes, it can be useful to measure the amount of water going into the tank as you add it.


With the water now in the tank, turn on the heater-thermostat and get the water up to your chosen temperature—salt will dissolve more quickly in warm water—and use a water pump or air diffuser to get the water moving, which will mix the salt.

Add Salt

Having either calculated or measured the volume of water, you can now work out how much salt you need—on the basis of 35 grams per liter of water or 4.7 ounces per US gallon. Weigh out the salt, and then add it to the heated, circulating water in the tank. Allow a few hours for the salt to dissolve, or leave it mixing overnight. It is worth noting that some salt mixes have a small quantity of insoluble residue, which usually looks like (and in some cases is) very fine sand. This is harmless.

Measure Salinity

Measure the salinity (it is useful to be able to read this as parts per thousand, rather than specific gravity, as this will make it easier to calculate quantities of water or salt to add if you need to make adjustments). If it is too low, you need to add more salt mix.

For example, if you have 100 liters (26.42 gallons) of water in the tank and the salinity is 33 parts per thousand (which is the same as 33 grams per liter), you need to add approximately 100 x 2 = 200 grams more salt to raise the salinity to 35 parts per thousand. The figure is approximate, because dissolving the salt you added already will have increased the volume in the tank, although not by much.

If your salinity is too high, you will need to add more water. Again, you can work out how much by comparing your desired salinity with the reading you have. If you get a salinity of 37 parts per thousand in 100 liters of water and you want to reduce this to 35 parts per thousand, you need to have a total volume of 37/35 x 100 liters of water. That works out to 105.7 liters, which means you need to add 5.7 extra liters of water. This might give you a problem if there isn't enough tank volume to fit the extra water in, but if you did leave some space for displacement, you should be okay.

Making Water Changes

When preparing water for water changes, the general principles are the same, but before you start, check the salinity and temperature in the aquarium and aim to match these parameters in the water that you make up. This is particularly important if you are making a large water change, as you need to avoid a sudden change in aquarium conditions.

The Most Crucial Element

Salt water may be the most basic element of the marine aquarium, but making it properly is crucial for a successful system. The most critical aspect, in most cases, is ensuring that the water going into the system is pure enough and then getting (and keeping) the salinity in the correct range.

Next month, we'll take a look at temperature control.