Do We Really Need Seawater Changes?

A while ago on a marine aquarists forum I saw a comment that the writer never did any seawater changes. The response was generally hostile with a few reasonable and helpful replies. I thought that the hostile replies were incorrect or at least too early. No question was put about how long the aquarist had been keeping a marine aquarium or why he thought that changes were unnecessary.

A new marine aquarist has quite a lot to learn by research which is usually on the internet and/or by reading books. It certainly isn’t difficult the marine aquarium hobby but some time needs to be given to education. This time is a commitment to the livestock that will eventually be kept and should help to ensure them a long and healthy life at the same time giving much pleasure and satisfaction to the aquarist.

Partial seawater changes are necessary and should be carried out initially weekly. The guideline for the amount changed is 10% of the total net gallonage including any sump. This guideline applies to the start of the aquarium and as the aquarist gains experience and learns the trends of his/her aquarium it can be adjusted up or down.

The source of the seawater is obvious, either purchased salt to be mixed or the natural stuff. The latter is fine if a sufficient amount can be transported successfully and an area of collection is known that is definitely not polluted. Once collected and at home the natural variety needs to be allowed to stand so that rubbish, minute or otherwise can sink to the bottom. The clean seawater is transferred and then heated and aerated before use. The majority of aquarists use the packaged variety, that is salt that needs to be mixed with fresh water then heated and aerated. The fact that most aquarists use manufactured salt is not a comment on the unsuitability of the natural stuff but the fact that most aquarists cannot access unpolluted seawater because of distance etc and/or do not have a means to transport it.

But why do we need to do these changes? The seawater in the aquarium usually looks clean and clear, the inmates seem to be happy. Have a look at the sea sometime, look at the huge area that can be seen and consider that this is just a tiny part of the whole. When looking at the sea, in your mind’s eye take your aquarium and put it in the area. Small or what! Really insignificant in volume.

In their natural habitat sea creatures have a huge volume of seawater available. Natural pollution is insignificant. In an aquarium the volume of seawater is very tiny in comparison and there is a price to pay for this. Pollution starts to build, partly from the sea creatures own metabolism and partly from the aquarist’s well meaning efforts such as feeding which leads to some pollution. In addition seawater contains trace elements and these reduce, a partial seawater change replaces these or at least partially.

As time goes on the trapped pollution increases. A good example of pollution is nitrate, something that rears its ugly head for some aquarists particularly beginners. Nitrate is produced from left over food and the natural breakdown of the creatures own processes, it also appears when the biological filter is housed in a powered canister. It can also be found in poor fresh water supplies used with manufactured salt. Nitrate is a food for algae which at the worst can ruin an aquarium with loads of unwanted algae everywhere.

So that’s it really – partial seawater changes are completed to protect the purity of the seawater and help maintain its composition. This is clearly a good thing for the aquarium inmates and also a good thing for the aquarist as it reduces the potential for problems.

Knowing how much seawater to change is easy. Start with the recommended 10%. Keep a chart and each time a nitrate test is done jot down the result, these results over a period will indicate if the changes can be slowly reduced or need to be increased. If there is a need to increase then pay attention to possible causes and take action. If they can be decreased keep monitoring until there is certainty of stability. Whatever the aquarist decides upon it is best to continue seawater changes if only to introduce partial replacement of trace elements and freshen the water.


  1. Hi John Cunningham

    In the 1980s I had an 80 gallon (tap water) marine aquarium with no sump (as they were not around then) and an Eheim submerged filter. I had a skimmer and trickle filter plus a section for a carbon bag and a primitive top-up box plus a water heater. All pumps were air-controlled. I had a few corals and fish with live rock. I had this set-up for ten years as I moved to down-size and passed the tank on to a relative. Two fish that I started with (a Percula Clownfish and a Pyjama wrasse) went back to the shop that I purchased them from. OK I was lucky! When I purchased this tank I did not used it for 18 months as I started to do my home work on marine set-ups. Yes in my spare time it took that long plus saving money for the fluorescent lights and such.

    In all that time I only done 10% WATER CHANGES ONCE A MONTH. Three days before the water change I filled a barrel with water (which was in the garage), heated it and mixed in the appropriate amount of salt and used an air-filter to air it. What was in that barrel was almost the same as what was in the tank.

    Take care
    Pete Page

  2. Hello Pete.
    How times have changed. I remember my early aquariums – dead white coral (much frowned on now and rightly so), the use of tap water as you mention, and getting really advanced with under gravel filtration! I did water changes about once a fortnight for the fish only system and everything seemed fine.
    How things have changed though. Most aquarists (this is a guess, I’m assuming most systems are reefs) have reefs and the need for high quality seawater is paramount because of the corals. All of the livestock in our care deserve and require high quality seawater, but it is the corals that really dictate. Seawater quality is effected by the fish load and for example the feeding habits of the aquarist (no, not cornflakes, how the fish are fed or overfed).
    I have to say that at my partial seawater change, done once a fortnight, the aquarium seawater has readings no different from the new seawater going in. However, this is stated from what I test for, the basics, so trace elements for example could well be dropping. Overall, it is better to do partial seawater changes even at the suggested minimum of 10%, freshening things up.
    Thanks so much for your comment.

  3. All of this begs the question – does nitrate evaporate? I have had tanks that had a lot of evaporation and the water quality was better.

  4. Nitrate doesn’t evaporate it just becomes more concentrated. Of course in a marine aquarium we have the anaerobic bacteria working away reducing it. We also do partial seawater changes to reduce it. With high evaporation there are more top ups and the water used is high quality, RO (reverse osmosis) for example. These increased top ups can also help to a very limited extent though this is general quality not nitrate reduction.

  5. Thank you very much and hope you writing more good articles.

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