Are Corals Best Kept On Their Own?

There are three types of marine aquarium and these are fish only, fish and corals (mixed reef) and corals only. If the aquarist has decided to keep corals only then there aren’t any questions to answer. The same obviously goes for a fish only system.

However, what if the aquarist goes for what is probably the most popular system, the mixed reef? What are the problems that could arise as far as the corals are concerned?

Generally speaking corals are more sensitive to poor seawater quality than fish meaning that the presence of fish is a negative from the start. Why should this be so?

Fish are generally the heaviest feeders in the aquarium and the larger the worse. Food and the end products of it cause seawater quality to deteriorate, for example the production of nitrate and the introduction of phosphate. The more fish the more pollutants. Poor seawater quality is combatted by routine seawater changes and good feeding discipline. Clearly, the aquarium should not be overstocked.

So all that needs to be done then is carry out routine seawater changes? The answer to this is yes of course, with a few more provisions. The guideline amount of seawater to be changed in the first months of the life of the system is 10% of the net gallonage (including any sump). The aquarist needs to carry out tests on a routine basis and keep a record of the results. There are several tests that are normally used for seawater testing, but the ones here are particularly nitrate and also phosphate. As already said nitrate is the end product of the decomposition of excess food and other organics and phosphate is mainly introduced with food.

Nitrate is a very useful indicator of seawater quality (but not the only one of course).Over a period of time the aquarist who has kept a note of test results should see a trend which will indicate the deterioration rate of the seawater (sometimes nitrate doesn’t appear or disappears after initial maturation). This trend will indicate to the aquarist the need or not to increase the amount of seawater routinely changed. If the nitrate continues to increase check the dry salt mix though nowadays this should not be the problem. It is also worth checking for nitrate content in the water being used to make the salt mix. 

If excessive phosphate is detected then again the dry salt should be checked though as said is not likely to be a problem. A check of the water used for the mix should be made. Phosphate is more likely to appear in a system containing fish.

So an aquarist with a corals only system is likely to find, all things being equal, that a seawater change of 10% is adequate. If there isn’t any indication of nitrate then it is worthwhile continuing with a 10% seawater change as this ‘freshens’ the seawater and adds some trace elements that are lost because of protein skimming as an example. Experienced aquarists could decide to reduce the seawater change amount a little, but routine seawater changes should not be abandoned. Of course, dependant on the type of corals kept other supplementation such as calcium could be needed.

There are few aquarists who are satisfied with just corals. For a start, the scene seems to appear more natural with fish present. However, once fish are present then seawater deterioration increases. How should the aquarist proceed?

In an ideal world the corals should be introduced first, permitting them to settle and also permitting the aquarist to be happy with the reef  layout. This procedure is not essential but is a good practice. After a few months when the corals are clearly healthy and settled the question of fish can be considered.

In a fish only system the stocking can be to the limit of the net gallonage (excluding any sump). This is not so in a mixed reef system. To assist with the maintenance of high quality seawater the stocking levels are restricted.

Again there is a ‘best practice’. There isn’t a requirement to have lots of fish as there already is a lovely and developing coral scene. Also the fish need to be fully compatible with the corals, it wouldn’t be pleasing if coral was a tasty lunch. They should not be too large, permitting them to move among the corals easily, small fish are usually the best and there are some really beautiful ones. The aquarist should add  two small fish first (hopefully after they have been quarantined). Once a minimum period of a month has passed all should be well with the fish settled into their new home. The aquarist should consider if his/her creation is sufficient, that is, is there a need for more fish? It could be that there isn’t. If there is, then add two more fish and again leave a month and re-consider. To repeat, there isn’t any need to have lots of fish. The movement of a few fish complement the already lovely reef scene. Under no circumstances should the maximum stocking guidelines be exceeded – and remember that fish grow. The stocking guideline for a mixed reef is a maximum 1″ (circa 2.5cm), excluding the tail, to 6 gallons of seawater, excluding the seawater in the sump. 

Using the above stocking method the aquarist is protecting the all important seawater quality by reducing the pollutants that are introduced into it. This in turn should reduce the size of the required routine seawater changes provided the fish numbers have been minimized. The result should be a mixed reef to be proud of.

  1. I agree with you – few people would be satisfied with a coral only tank. However, I personally find them beautiful and have one. I also have another where I just keep a clown fish because they are supposed to help.

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  13. Fish are generally the heaviest feeders in the aquarium and the larger the worse. Food and the end products of it cause seawater quality to deteriorate, for example the production of nitrate and the introduction of phosphate.

  14. Yes that’s correct. In order of what the highest quality seawater should be, that is, the easiest to maintain first:
    The Coral Only system. There aren’t any fish.
    The Mixed Reef system. There aren’t that many fish.
    The Fish Only system. Often mamimum stocking.
    Of course, any system can become polluted.

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