It’s Number One. It’s Seawater.

There are many parts to a marine system and much time is spent researching to get the equipment right. Once the system is together more time is spent trying to ensure that the livestock is compatible with the aquarium size and system type and also they’re happy with each other. There is one part of the system that creates more trouble than any other and that is the seawater.

Assuming that the system is well thought out and set up and also that stocking is correct it could be thought that everything will now be relatively straightforward. There’s routine maintenance to be completed and everything should be rosy. So it should be and there should be many an hour spent watching the aquarium. This is proper progression but unfortunately there could be problems, even major ones that arise particularly with newcomers to the marine hobby. Much of this trouble is with the seawater.

High quality seawater is the number one requirement whether the marine system is mixed reef, corals only or fish only. The need for high quality is because the livestock are constantly in touch with it, breathe from it and depend on it. If the quality reduces excessively then corals will close and fish will begin to lose their bright colours and vitality.

The highest seawater quality is required with coral only and mixed reef systems as corals will not tolerate reduced quality well. The coral only system seawater is fortunately the easiest to maintain, followed by mixed reef and fish only in that order. So there’s the clue. It’s mainly to do with fish.

Corals do not have a major impact on seawater quality but fish do and the higher the number the greater the impact. Once fish are placed in the aquarium the seawater begins to deteriorate. This is because of the natural bodily processes of the fish. Therefore it’s important not to overstock and the reason maximum fish stocking guidelines exist. The stocking guidelines suggest a lower fish loading for a mixed reef system than a fish only one to protect the needs of the corals.

Fish also have another requirement that brings pressure to bear on seawater quality. Like all living things they need food. This food can be anything from de-frozen fish to flake. Many beginners overfeed as they are concerned that the fish get enough. This attitude is to be applauded of course, but sadly it often leads to trouble. There could be excessive algae or even an algae invasion where everything becomes coated with the stuff. Some algae are good and decorative and welcome, but not the ones that cover with a green hairy coating. It is probable that the great majority of marine aquarists, experienced or not, overfeed their fish to an extent because it is nearly impossible to ensure that every morsel is eaten. Some of it goes into rocks for example. However, particularly in very mature aquariums it’s likely that the tiny life forms that dwell in the rocks benefit from this small overfeed and reduce its impact on the seawater.

So the aquarist wants a vibrant healthy and colourful aquarium. The livestock want high quality seawater (corals of course want suitable lighting which is a close second to seawater quality). A happy aquarium and aquarist are quite easily achievable.

Whatever type of seawater that is used in the aquarium – natural or mixed from dry powder – it will deteriorate. The first action to combat this is to carry out routine seawater changes. The guideline suggests 10% of the net gallonage (including any sump) weekly. This can be varied once the aquarist knows the seawater trends but routine changes should continue even if decreased. Completing routine seawater changes removes some unwanted nitrate for example and freshens the seawater introducing some replacement trace elements.

The second action is to test. The major tests are:

pH to ensure the alkalinity of the seawater is stable and as it should be. The natural trend for seawater in an aquarium is to acidify which means the pH goes down. The pH should be checked in daylight, meaning the lights are on, and produce a reading of between 8.0 and 8.4. The readings shouldn’t vary with these figures but should be reasonably stable on one.

Salinity should be stable and have little variance. The measurement could change because of salt creep or evaporation. With small systems manual daily top-ups should suffice. Larger aquariums could make use of automatic topping up mechanisms. The guidelines suggest for systems containing corals a measurement of 1.024 to 1.026 and for fish only systems 1.020 to 1.022. The lower one for fish is because it’s believed that some fish parasites do less well at these lower figures. However, fish will not suffer at the higher figures given for corals.

Ammonia and nitrite are very dangerous and can kill. Therefore the readings for both should always be zero. It’s important the bio-filter is properly matured to ensure the bacteria are able to deal with these toxins.

Nitrate in excess is bad as it is a known nutrient for nuisance algae – the horrible hairy green stuff and others. The guidelines suggest 30ppm (parts per million) or less for fish only systems and 10ppm or less for those containing corals. The nitrate levels should be as low as possible.

Temperature has an effect on the metabolism of the livestock in general. The reading should be stable and the guideline suggests between 75 and 80degF.

Feeding has already been mentioned. Feeding the fish is a pleasure and it’s good that it is. However, the fish will eat more than they need – in the wild they don’t know when the next meal will be found so they will stuff in as much as they can. It’s believed that they could even   excrete half digested food in order to do this. Once fish are settled in the aquarium they will often rise begging for food and the inexperienced could believe they need more. Excess food is going to lower the seawater quality so overfeeding needs to be avoided. Resist any begging and cease to feed as soon as the fish show signs of a reduced desire to eat.

Seawater quality could include other tests particularly in some coral only and mixed reef systems. Consideration of these has been excluded as the aforementioned forms the major basis for high quality unpolluted seawater. The aquarium and aquarist should be happy once the discipline of seawater quality maintenance has been achieved.       


  1. The above text is similar to some already written – however seawater quality continues to be a problem area with beginners so I did it again!

  2. good info. I feel that you have describe the balance of the saltwater, coral and the fish in layman’s terms so that everyone should be able to understand it.

  3. Thanks for that. The hope is that the information on this website will help to de-mystify the marine hobby for the beginner. Procedures are generally quite straightforward.

  4. Thanks a lot for the well written article, seawater aquariums are harder to maintain than freshwater aquariums but seawater creatures are more beautiful and colourful.

  5. Marine aquariums are more difficult than freshwater generally speaking but basically not that difficult. It’s unfortunate that quite a few potential marine aquarists are put off by the seemingly difficult procedures and the varying opinions on forums some of which are misleading. Marine fish and corals are very colourful, more so than freshwater life – but then I’m biased!

  6. I just couldn’t depart your site prior to suggesting that I extremely enjoyed the standard information an individual provide for your visitors? Is gonna be back frequently in order to inspect new posts

  7. This site would be a must read for anyone who is thinking of setting up an aquarium with sea water.

  8. It’s very much hoped that beginners to the marine hobby and more seasoned aquarists also will find this site useful, and find guidelines on the various considerations involved to achieve success.

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