What you need to know about marine salt

The fundamental difference between freshwater and marine tanks is of course salt. If you’re new to keeping marines, or are considering moving over to the salty side of the hobby, here’s everything you need to know. 

It’s not just any old salt!

Salt is salt. Right? Not exactly. Salt as we know it is Sodium Chloride, NaCl, but the “salt” in the ocean is actually a chemical soup containing dozens of elements including calcium, magnesium, potassium, even mercury and arsenic! And of those elements, about thirty are important to corals and invertebrates. 

So when keeping marines you need marine specific salt – not table salt, which is refined and contains anti caking agent, and not pond or tonic salt, which is designed to be used in small amounts as a beneficial additive for sick freshwater fish.

Only marine aquarium salt contains everything that tropical marine fish, corals and invertebrates need in order to stay healthy, and thrive in captivity. 

Types of marine salt

Marine salts can be divided into two main types – synthetic, and evaporative. Synthetic salts are made up of mined ingredients, formulated and mixed in a lab, and evaporative salts are made from natural seawater which has had all the freshwater removed by desalination methods, leaving the dry salt mix behind. 

Both types have benefits, and both are widely available, and give proven results. To look at the salts themselves you wouldn’t notice the difference, as both are very fine, clean, white, powdered salts, ready for mixing. So whether you opt for evaporative or synthetic is down to personal choice, and many users wouldn’t notice any difference. 

Elevated levels

Salts can be further divided into those which are designed to replicate natural seawater parameters, and those which have been boosted to have higher properties of alkalinity (KH), Calcium and Magnesium. Salts with elevated levels are often referred to as Reef or Pro recipes, although both are equally suitable, and again it is down to personal preference. Reef tanks don’t have to run on reef recipe salts, as long as parameters are boosted long term in other ways, like from dosing.

What you need to get started

Salt is available in many shapes and sizes, from small bags to large buckets. The most important first consideration is to make sure that you have enough to fill your tank. Salt packaging will state both weight in kilograms and how many litres of salt water it will produce when mixed with fresh water, so if you have a 250 litre aquarium, you will need enough salt to make up at least 250 litres of saltwater, just to fill it for the first time. 

You’ll also need a clean bucket or spare aquarium, a heater, thermometer, and a pump to circulate and mix the salt into the water. An airpump and airstone could be used, as well as or instead of a water pump, to help to mix the salt and circulate the water.

The directions on the salt packaging will tell you how much salt you need to add to mix seawater of the correct salinity. The amount of salt you use each time does actually vary from manufacturer to manufacturer, as some are drier, and more concentrated than others, so always test salinity during and after mixing.

RO water

Tapwater contains nitrates and phosphates, which you don’t want to introduce to your aquarium. Always use Reverse Osmosis (RO) or Deionised (DI) water, which should be largely free of nitrate and phosphate, and other pollutants. Purchase an RO unit you can make up your own water change and top-up water from home – two essentials when keeping marines. RO/DI units combined are the best possible form of water purification for aquariums. They’re performance can be monitored with a TDS meter (Total Dissolved Solids.) 

Salt testing

The saltiness, or salinity of your freshly made seawater should be mesaured by a Hydrometer or Refractometer. Hydrometers are cheap and easy to use, and often chosen by beginners, but Refractometers are more accurate, and often chosen in preference by experienced reefkeepers. Both are calibrated to give an accurate reading only when the water has been warmed to 25C, and will give a different reading if the water is cold. Refractometers can also be calibrated by the user, to make sure that they are always accurate and reliable. Any reef aquarium water test should always start with an accurate salinity reading, as if the water is not mixed to the right strength, other values like KH, Calcium and Magnesium will also be too high or too low. 

Ready mixed 

Both synthetic and natural seawater are available to purchase ready-mixed. The manufacturer has removed all the hassle of mixing and testing your own saltwater, and as long as its not freezing cold, it can be poured straight into your tank. Ready-mixed saltwater works out more expensive per litre than mixing your own, but it is the ultimate in convenience, and can be delivered, ready to use, straight to your door.   

Probiotic salts

Some salt manufacturers also produce Probiotic salts. These include beneficial bacteria which can potentially help to reduce nitrate and phosphate levels in reef tanks if used regularly, although most are only used by coral specialists, and with demanding corals like sps corals, where nutrient and coral colour management is the priority. 

Storage

Always keep marine salt in a dry place, away from any moisture. Marine salts are anhydrous, meaning they are totally devoid of any water in the packaging. If they are left exposed to the air, they will absorb water from the atmosphere, and the salt will clump. If you use salt infrequently, buy several small packages, keeping it sealed and unopened until you need it.   

Ongoing saltwater maintenance

Unfortunately, just adding saltwater to your marine aquarium and leaving it is not possible. Saltwater evaporates, removing freshwater and leaving the salt behind. If left, marine aquarium water will become saltier and saltier, and pumps will start to draw in air and run dry. All marine aquariums should be fitted with an Auto Top Off device (ATO) to automatically top-up water lost through evaporation with fresh. ATOs can be used on marine aquariums with or without sumps, and with or without lids.

If you have corals in your tank and they are healthy and growing, corals will deplete the elements dissolved in the saltwater, like KH, Calcium and Magnesium. To counteract this degrading of the salt, change water regularly or test for and add supplements, either daily by hand, or via an automatic doser.       

FAQs

Can I swap from evaporative to synthetic salt, and vice versa? 

Yes, you can swap salt at any point and even go from synthetic to evaporative, and back again, with no problems. Just mix the new salt in the normal way and start to introduce it at your next water change. Monitor your corals and water parameters over the next few weeks and months and see if there are any positive changes, and if you want to change to something entirely new again, you can. 

Can you give me some examples of synthetic and evaporative salts?

D-D and Red Sea are made from evaporative processes, with water taken directly from the Red Sea. Tropic Marin, Aquaforest, Seachem, Instant Ocean, Reef Crystals and Fritz are all fully synthetic salts. 

How quickly can I add the saltwater after mixing? 

If the water is the right temperature, the right salinity, and all the visible white salt has dissolved, you can use it. This may happen in as quickly as a few hours after mixing, or it may need to be mixed overnight. Give the water in the bucket and the water in the main tank one last check before pouring it in. If the water looks swirly when you add it, it’s a sure indication that the salinity of the water in the tank and the bucket are different.   

Can I collect my own natural seawater?

We don’t recommend that you use the seawater from around our coastline. It differs slightly from the pristine waters found around tropical coral reefs, and may contain pollutants or temperate plankton strains. Much better to buy specific salt or ready-mixed seawater, free of pollutants, and with proven results.  

My parameters are different to those stated on the salt packaging?

Salt manufacturers use very accurate analysis equipment to test their own salt values. Check that your water is the right temperature, and same salinity, as that stated in the product analysis. Check that your test kits are in date, and your equipment calibrated, and give the dry salt a good mix and shake before using, to avoid settlement.         

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Jeremy Gay is an author and freelance aquatic specialist. A former editor of Practical Fishkeeping magazine, he offers a wealth of experience on all things aquarium and pond.


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