Natural Seawater – Is It Recommended?

Mother Nature provides the habitat for the livestock that we aquarists keep in our fish only aquariums or reef aquariums, and this, of course, is the wild reef. Mother Nature is also the provider of the other obvious essential, and that is seawater.

Having written the above it would seem absolutely wrong to state that natural seawater is not recommended. So it would be – natural seawater is recommended.

However, is it recommended overall, for the majority of aquarists? This needs a bit more care, and two questions arise. These are first, the ability to collect the seawater, and, second, before any attempt is made to collect it, what is the seawater’s condition?

Not all aquarists live within easy travelling distance from the sea, but there are a fair number where it could be a viable proposition. So in this case the first consideration is practicality.

Seawater is heavy, it ways somewhere towards 10lbs per gallon. So to do, say, a 10% water change a good few gallons are required. Not so many for a nano aquarium, but increasingly more as the aquarium gets larger. Also of course, some aquarists do water changes that are more than 10%.

This water has to be moved from the sea to the aquarist’s home. This means there needs to be sufficient containers of seawater safe construction which are also strong enough for the journey. In addition there must be transport that is robust enough, and roomy enough, to carry the full containers.

If the aquarist cannot meet these requirements then the obvious answer is to use one of the available commercial dry salt mixes. Transportation problems for these mixes are zero as they come in different size packs, can be transported by car, or can even be delivered to the aquarist’s home.

If the aquarist can meet the transportation need, then another consideration arises. This concerns the question of pollution. It is very unfortunate that many coastal areas are polluted with industrial waste and/or agricultural run-off and the like. The aquarist must be absolutely sure that the seawater being collected is clean. There might be a facility nearby who could advise on this matter. If not, it is better to play safe and use a dry salt mix.

If the coastal water is polluted, then natural seawater could still be used. By going considerably offshore, it is possible or maybe probable that the seawater will be unpolluted – but not definitely so. Again, the aquarist must be certain of the condition.

Going offshore brings the transportation problem back. Is there a boat available? If the aquarist is also a yachtsman all is well and good. If not, the only means is to hire assistance, and then cost is creeping in. In the latter case, the effort may not be financially worthwhile, so again a dry salt mix could be the preferred option.

Using natural seawater brings up another possibility, and that is disease or the introduction of unwanted life. This applies particularly if the natural seawater being obtained is warm.

This danger can be minimised. The collected seawater can be kept in the containers in the dark for a week or better two, and at the same time have it passing through UV (ultraviolet) lamps. These can be obtained specifically for aquarium use. If there were more than one container, they would have to be connected or the aquarist would need more than one UV lamp. After the mentioned period, the seawater could be used after where necessary being heated to the required temperature, and after any sediment that had accumulated on the bottom of the container(s) had been siphoned out.

There is one more consideration. This is that the seawater could still need supplementation. Just because the seawater is natural does not mean that in a reef aquarium there will not be a need for calcium addition and the like. In the confines of the aquarium, a heavy demand will have the same effect as it would if synthetic seawater were in use.

Some aquarists have reported that using natural seawater has a wonderful impact on their captive reefs. This is of course anecdotal but there isn’t any reason not to accept the reports. Perhaps there is some ‘magic’ in natural seawater that makes a difference?

Looking at so many captive reefs that use commercial salt and can be viewed on the internet and in reality, and considering the great success and beauty of these reefs, there isn’t any real need to doubt modern commercial dry salt mixes.

If the aquarist can meet the demands for using natural seawater, then all is well. Most aquarists will be happy using commercial salts.


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7 Comments
  1. Go commercial salts:) Never let me down!

  2. I only use original seawater, because I don´t want to risk something.

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  4. Thanks and really glad it is useful and enjoyable.

  5. Thanks for providing such a useful web site.

    My experience:

    Every 6 months or so, the tank starts to look drab, or the most sensitive corals start dying. Then I go to the sea, and collect fresh seawater.
    It’s fun and good exercise.

    When I do a water change with fresh (Mediterranean) seawater, the corals really perk up! I use fresh seawater, which includes plankton.

    There is a risk of introducing parasites or disease. I have accidentally introduced copepods, but fortunately they were apparently harmless. Now I look for them in the buckets with a bright light, and I take care not to introduce them. I also try to exclude floating or sinking garbage, such as bits of plastic or wood. It would be safer to completely filter the water first, but I take the risk since I only have one fish. Although the water contains foreign floating objects, it clearly supports life very well, probably much better than synthetic mixes (I have used both).

    Happy reefing!

    Meir 🙂

  6. Hello Meir. Unsurprisngly the positives for Mother Nature’s seawater (NSW) continue. I assume you’re doing about a 6 monthly routine partial seawater change and action this when some corals appear to be dying or at least looking drab. I would suggest that the period needs decreasing, so that the more regular partial seawater change prevents the rundown with the corals in the first place, and there’ll be no period required for them to recover. It could also mean a smaller routine change.
    Some NSW users use UV filtration but of course this would damage or kill the plankton you desire.
    Thanks for your comment, very welcome.

  7. NB: Each body of water has different salinity. For example, the Mediterranean Sea has unusually high salinity, and it is colder than tropical reefs. I have found the higher salinity very suitable for the corals and invertebrates (and the fish do fine also).

    Make sure you adjust the salinity and temperature to match the actual conditions of your reef aquarium to avoid shocking the inhabitants. In case of emergency (if the tank is polluted for example, and needs an instant water change), I have found my (non-SPS) corals respond well to the fresh seawater, even if it was not adjusted.

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