The Challenges of Collecting Natural Sea Water for Your Marine Aquarium

Collecting natural sea water for your aquarium can present a number of unique challenges

Collecting natural sea water for your aquarium can present a number of unique challenges

Given the ready availability—and relative affordability—of high-quality synthetic sea salt mixes, it’s not surprising that the vast majority of marine aquarium hobbyists today choose to use artificial sea water rather than collect the natural stuff from the ocean for their systems.

But that doesn’t necessarily mean synthetic salt water is superior to its natural counterpart in all cases. Depending on a hobbyist’s circumstances, collecting water from the ocean can be a perfectly viable alternative. In fact, some hobbyists who use natural sea water in their tanks absolutely rave about the results they see in their livestock.

Obviously, for people like me and Chris, who are located many miles from the nearest ocean, collecting natural sea water is a lot like failure on the Apollo 13 mission—not an option. But for salties lucky enough to live right on the coast, it may be a choice worth exploring.

However, there’s no question that there are certain challenges attendant upon collecting natural sea water. If you’re one of the fortunate few “coastal salties” (or as Chris and I like to call them, “long-lost relatives”), you’ll have to decide for yourself whether any of the following obstacles are deal breakers:

Finding a clean source

There’s plenty of clean water in the ocean, but it’s not always as easy to access as we’d like. The inshore waters you can reach with the least amount of difficulty—e.g., from a pier, dock, boardwalk, boat launch, or beach—are also the most likely to be polluted with marine fuel residue, agricultural runoff, sewage, and other contaminants. That’s why it’s best to collect as far off shore as is practical.

If you happen to own a boat (or know someone else who does and is willing to take you on regular water-collecting trips), traveling off shore to collect non-polluted water may not be a major problem, but if you don’t, you’ll need to find a site within reach of shore that’s less likely to be contaminated. That means an area with:

  • Minimal boat traffic
  • No rivers, streams, or creeks discharging nearby
  • No nearby urban or industrial area
  • Stable, appropriate salinity (i.e., not an estuary or other brackish environment)
  • A healthy population of marine wildlife

Also, if you must collect close to shore, in addition to the points listed above, try to do so when the incoming tide carries in the cleanest possible water.

Heavy hauling

There’s no getting around the fact that collecting natural sea water involves some heavy lifting and logistical challenges—especially if you’re collecting for a large aquarium. Five-gallon buckets, heavy-duty plastic bins or trash cans, and similar watertight vessels are the usual tools of the trade for hobbyists who collect their own sea water. Once these containers are filled, lugging them to your vehicle (possibly after hoisting them off a boat) and then into your home can be a real chore.

Treating the water

When mixing artificial salt water using RO/DI-purified water and a quality sea salt mix, all you have to do is make sure the basic parameters—specific gravity, temperature, pH, etc.—are in the correct ranges before using it in your aquarium.

Natural sea water, on the other hand, tends to require a lot more pretreatment before it’s safe to use in aquariums because of the dissolved organics and planktonic life it often contains. This typically involves filtering the water through a micron filter sock at the time of collection to eliminate as much debris and plankton as possible. Then, processing of the water usually continues once the hobbyist gets it home in order to render it safe for aquarium use. This might involve one or more of the following steps:

  • Storing the water in a dark container for at least a few weeks to allow any remaining plankton to die off and settle to the bottom, and to allow bacteria time to consume any dissolved organics
  • Running the water through an ultraviolet sterilizer to kill off any parasites or disease-causing microorganisms
  • Treating the water with chlorine to kill off any life form, followed by the use of a dechlorinator

Of course, after that, it’s still necessary to test and possibly adjust all the critical water parameters to make sure they match the parameters in your aquarium.

Do you collect your own salt water?
If you’re a coastal salty and collect your own sea water, we’d love to hear how you’ve overcome the logistical challenges and how it’s working out in your system. Let us know in the comment section below!

Photo Credit: Matthias Rhomberg


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About Jeff Kurtz

Jeff Kurtz is the Co-founder/Editor of Saltwater Smarts, former Senior Consulting Editor for Tropical Fish Hobbyist Magazine, and the aquarist formerly known as “The Salt Creep.” He has been an aquarium hobbyist for over 30 years and is an avid scuba diver.


  1. Nothing is compulsory, it is a tool, but a very good tool

  2. Hey guys! I recently collected live rocks from the Red Sea and noticed brown hairy organisms all over the surfaces and I’m wondering what are these (look like dead green hairy algae) and how can I get rid of them. And by the way, I also collected natural sea water and started cycling it on a new 30 gallon tank.

    Here’s a link to the photo:

    I’m looking forward to your feedback and many thanks in advance.

  3. The hairy stuff would not bother me but those grape looking algae’s I believe would be invasive although I am not an expert on Red Sea flora. If you keep that rock in complete darkness for a few weeks, in water that should kill all the algae and keep the bacteria.
    I would put it in my tank the way it is but that is an experiment that I would not advise any one else to do.

  4. Hi Noel, I am a marine biologist for ARC Reef. We sell aquaculture live rock I can identify about 5 different species of macro algae as well as diatoms on those rocks. They are all fairly slow growing and not likely to take over your tank. The diatoms are a natural part of the cycling of your tank. I can tell that you took those rocks very close to shore where nutrient levels are much higher than if you took them from on a coral reef. It’s not desirable to have them in your tank as the goal of a reef tank is to have an ultra low nutrient level. I can tell you that they will die off though in a matter of about 2-3 weeks or slightly sooner if you have a few crabs in the tank. I have dove on the Red Sea and the bay of Aqaba and it truly is an amazing reef with a spectacular density of life. Please be careful in taking any rocks from the sea though. Nearly all countries have strict laws prohibiting such activities. In the US the fine may also include actual prison time. So please be sure to look up your countries laws prior to taking any live rock or corals from the ocean. My suggestion is to add a variety of hermit crabs to your tank. A couple red foot and blue foot hermits will munch on those macro algae in a matter of a week. If you get any nuisance algae I would recommend reducing the photo period to 6 hours or less. If you eliminate all light you will kill all algae and bacteria and raise your ammonia and nitrite levels, essentially starting another tank cycle. Also be aware that the red Sea has slightly higher levels of salinity. So i would properly aclimate them to a specific gravity 1.026 over the period of a day or even 2, depending on what area of the Red Sea that they originate from. I hope this information helps you out. Best of luck to you and your tank!

    • That’s a lot of information. Many many thanks! We’re actually planning to acquire crustaceans this weekend from a local marine aquarium shop for that purpose. I have two crabs (hitchhikers) but they burrowed under and would not climb the rocks to help cleanup.

      I’m also looking at your page and this one I have to read as well:

      Thanks again!

    • For Hermits should add a variety of (empty) shells for them to utilize as they grow? I love in Okinawa and Im really looking to starting a Natural reef tank and pretty much have all natural resources available in all directions literally minutes away, so I’m super excited to say the least.

      ARC, can you expound on: “f you eliminate all light you will kill all algae and bacteria and raise your ammonia and nitrite levels, essentially starting another tank cycle.”



      • Sure Brian, our facility is located on the bay and we used to pump in and out seawater. If you strain the seawater that makes it past a 100 micron filter to a 1 micron filter and then run it though a centrifuge you will see that there is quite a bit of life still remaining in that prefiltered water. We could fill a 15ml testtube of organisms for each 5 gallons that were spun in our centrifuges. If you were to then shut the lights off on the collected water those organisms, bacteria, alga, tetra spores etc will die and any death can cause ammonia. We even found hydroids in our collection tubes that made it past a 1 micron filter and then grew. 10 years ago I would say this was the best method of collecting water for tanks due to high priced salts and their inconsistencies. As of a couple years ago our facility went 100% closed system as did other commercial facilities (seaworld, miami seaquarium, mote marine laboratory, Rosenstein, NOAA) We culture and sell coralline algae in a bottle, a product that you may have heard of, coralline algae is very sensitive to many organisms and salinity change. Our facility is massive, on 2 acres of land but we have determined that due to the now low cost of salts ($50 per 200 gallons) that the benifits of not collecting fresh seawater outweigh the the cost savings of pumping in free seawater. If saltmix is not available in your geographical location then collecting on the beginning of a slack high tide will still work in a pinch, filter with a 1-5 micron filter, use an air pump or powerhead to keep motion in the collection bin and add a UV filter, basically treat the collection tank as a reef tank and think of the water as being alive and not simply a bucket of tap water with no life in it. You will also still need to adjust the salinity, most seawater from the bay or within 5 miles of a river inlet will have lower than ideal salinity but if you find corals growing nearby such as how corals will grow right on a dock in the Bahamas or red sea then that is a good sign. Also for adding crabs it is best to add a variety, (scarlet, blue, brown, etc) different hermit crabs eat different nuisance alga, and sometimes it depends on the individual crab himself on what he will eat. Do not listen to fish stores on the 1 crab 1 snail per gallon rule, they will starve to death very shortly if you put 25 hermits and 25 snails in a 25 gallon tank. Adding extra shells is needed also, hermit crabs are cute but they are all murderers, they will all fight and try to steal every snail and hermit shell they can to the point where if you had 25, only 5 alphas would remain eventually. Owning an aquarium is one of the most rewarding experiences but you are responsible for the lives of those animals that are under your care. When possible please remember to choose aquacultured species or captive bred. This will minimize the impact that our hobby has and will ensure that future generations may enjoy and learn from it just as we did. Hope this helps and Happy Reefing!
        ARC Reef
        Atlantic Reef Conservation

  5. Arc Reefs, thanks for responding.

  6. Bruce Jordaan says

    Hi, Have been using natural sea water to fill my tanks and for water changes for over 40 years and never once did any of the things mentioned above. Collect the water and put it straight into the tank.
    Have never had any side effects from doing it the way I do in fact my father had marine tanks way back in the 50’s. in South Africa.

    Regards Bruce

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