Is Seawater Quality Affected By Fish And Corals To The Same Extent?

Seawater quality is the number one requirement for a marine aquarium. The close number two for corals is suitable lighting. Given that both of these needs have been met where both fish and corals are present then the aquarist should have a beautiful healthy aquarium population.

How can such a general statement be made? Surely there is more to success than just seawater quality (and lighting). There is and some of these are seawater movement, correct feeding, correct stocking, properly functioning equipment such as the protein skimmer etc. Think about it a little and it’s realised that the measure of the adequacy of stocking, the equipment, feeding etc is the seawater quality. Overstocking, overfeeding, an inadequate protein skimmer, insufficient seawater movement – they’ll all tend to show up in seawater quality reduction.

There are three types of marine aquarium – fish only, corals only and mixed reef. The latter seems to be the most popular and this is understandable as it comes closest to Mother Nature’s own though with a massively reduced size. All these aquarium types have their advantages.

With a fish only system stocking can be to a higher level. Seawater quality is still important but there is more tolerance. For example, the guideline suggests nitrate at 30ppm (parts per million) or less is acceptable, the lower the better. With a coral only system the picture changes as the guideline suggests 10ppm is the maximum again with lower being better. The same goes for a mixed reef, 10ppm or less.

If a fish only system is fully stocked then the effort to keep nitrate (for example) low is still a challenge. Fish demand food and the more fish the more food. This produces nitrate so the same effort to avoid overstocking and overfeeding is needed.

A corals only system is, all things being equal, the easiest to keep. Why? Corals do not put the same level of bio-load on the system that fish do, it’s much lower. Therefore the effort to keep nitrate low (for example) is considerably less. This means that seawater routine changes could be of a less amount, though they are still required. It should also be remembered that with some corals it’s likely to be particularly necessary to add supplements to the seawater to maintain adequate levels. An example of a supplement is calcium. More than one supplement could be required depending on the coral’s needs.

The mixed reef system is the most difficult. This is because the guideline suggests the needs of the corals as regards seawater quality is the target, not the fish. Therefore the guideline suggests that the stocking level limit is around half that of a fish only system. Some aquarists put in less fish than the guideline suggests purely for the benefit of seawater quality. Even so, great care needs to be taken with feeding, and there could still be the need for supplements such as calcium. Depending on the corals kept, the aquarist could be required to provide several supplements for a mixed reef aquarium.

Though the acceptable pollution level is different, all of the aquarium types require high quality seawater in relation to their population. It can clearly be seen that the higher pollution potential is with fish. That is why the mixed reef system seawater is likely to be more difficult to maintain as the quality target is for the corals with fish present.

Provided a coral has the room to expand and has the necessary requirements included in seawater quality (eg. calcium and seawater movement) plus adequate lighting, it will be happy in an aquarium that is only just big enough to contain it. Pollutants will not increase rapidly though they will increase over time – routine seawater changes would still be required.

A fish cannot be placed in an aquarium just big enough to contain it. It would be cruel as the fish would have an inadequate area to move in. As far as seawater quality is concerned, the fish would need feeding and pollution would increase rapidly. This is why the ‘fish per gallon’ guidelines exist for both fish only and mixed reef systems.

Going back to the real world, it is important that stock is carefully selected for fish only or mixed reef systems. (The same goes for corals only of course but as pollution is less of a problem the system will be excluded.) Not only should fish be selected for type and potential size bearing in mind the size of the aquarium, fish should also be selected that will not damage each other or corals. Corals also should be selected so that their potential size (remember expansion) will not cause them to touch, particularly unrelated corals. Despite their beautiful appearance, corals can be aggressive and could fight which causes considerable damage. Selection, particularly of fish with regard to seawater quality, should include feeding type so that food is known to be available and offerings are less likely to stay and rot in the aquarium. It’s impossible to ensure that all food particularly flake types are taken as some finds its way into the rocks. In very mature systems the little life forms that have evaded predation could well reduce this pollution but nevertheless lost food is not desirable.

So seawater quality is not affected to the same extent by fish and corals. Fish have a much higher impact than corals. With fish present the bacteria factory so essential to aquarium health which operates the nitrogen cycle has to work harder with more workers. The potential end result of this is more pollution.

So next time the urge comes to add ‘just one more fish’……don’t!

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2 Comments
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