Introducing Corals To The Aquarium

Owning a reef aquarium is the ambition of most marine aquarists and understandably so. There are those who have a fish only system as they believe it imposes less work and discipline than corals but this is not wholly correct. It’s true that there are disciplines required but not a great deal more than for fish.

A reef aquarium does not replicate nature’s reef of course as it’s too small and does not have the huge range of lifeforms. However, correctly set up and cared for and given some time for the corals and fish to settle, it presents a picture that is very reefy. Given more time many tiny creatures could find a home after being introduced with live rock.

A reef system can be created from an existing fish only system or from new. Let’s take the fish only system first. The owner of the system will recall the care required to obtain compatible fish and this care has to be extended – it is quite likely that some of the fish already in the aquarium will need to go. This will not be a popular move but is necessary as compatibility is not the only issue. A fish only system could hold around twice as many fish (subject to the constraint of eventual size) as can a reef. This is because of seawater quality. Fish, speaking generally as there are exceptions, do not need such a high seawater standard as corals.

So a choice has to be made – which fish are to go? To help with this a check of fish/coral compatibility needs to be made. Some fish are definitely not for the reef system and if so they must go. This could achieve the desired reduction but if not, well, grit the teeth and choose more, particularly those that may be ok with corals. If they go, the ‘may be’ disappears. The local dealer and other aquarists are often helpful with fish re-homing.

The fish are now at the number desired. Now the suitability of the aquarium decor and the current seawater need to be considered. Many fish only systems contain rock which could be in the form of a reef. If the rock is porous and mature all the better as the nitrogen cycle will be present, or rather the microorganisms that operate it. The nitrogen cycle is the process of poison removal: ammonia to nitrite and nitrite to nitrate. The first two are very dangerous and the last an undesirable. If the rock was purchased as live rock all the better. If not, it needs to have been present for many months or a year so that the necessary microorganisms are present. If the system is being run without the aid of a power filter etc then all should be well (the seawater needs to be tested carefully more than once). If only a few rocks are present then new live rock should be introduced to boost the cleaning abilities of the microorganisms and increase the ease of coral placement. Live rock is the best for a reef. A power filter can be used, but care on seawater quality is required as a power filter will not assist in the removal of nitrate. This is because nitrate removing organisms require a habitat very low in oxygen, they obtain their oxygen by extracting it from the nitrate thus breaking the nitrate down – a power filter is pumping oxygenated seawater through the filter. Seawater quality must be monitored particularly with a power filter (or similar filter using oxygenated seawater) to ensure nitrate levels do not rise. The suggested limit for nitrate in a reef aquarium is 10 ppm (parts per million).

Ok, so the fish only system is ready to become a reef, or is it? What about the lighting and also seawater movement? The fish only system could have, say two fluorescent tubes. This is not enough. Provided the aquarium is not too deep fluorescent lighting is fine, put in as many tubes as can be fitted. These should be a mix of blue actinic and marine white. They are easy to identify at the dealers or on line, as they are usually described as marine blue or white. If the number of tubes fitted is odd make the final tube a white. Fit the tubes alternately by colour. It is a good idea, if not already present, to fit timers so that the blues come on about 30 minutes before the whites and turn off 30 minutes after. This gives a dawn/dusk effect and fish do get used to it.

This leaves the seawater movement. Not a problem, there will be powerheads etc already present. The suggested movement for a reef is twenty times or more per hour. The capability of the existing pumps is known, are they powerful enough? If not they can be upgraded. The fish only system is now ready to become a reef.

What about the new aquarium? There will be a need to purchase heater(s), a protein skimmer and a thermometer. Excluding fish everything already mentioned applies. Reef creation out of live rock is the best as this avoids the need to mature other rocks. Lighting can be fluorescent as described, there are other lighting systems available for deep aquariums. Powerheads can be obtained which will supply sufficient seawater movement. There is a need to purchase test equipment, the minimum is an hydrometer plus pH, ammonia, nitrite and nitrate tests.

There is one difference – it is best to introduce fish after the corals for two reasons. First the fish will not be disturbed if changes to the reef rock positions are required because of stability or the placing of corals and second the corals will be settled. This ‘corals first’ suggestion is not a rigid requirement but can be helpful and also assist with that needed quality for marine aquarists, patience.

Seawater quality must be monitored and should be stable with little nitrate and no sign of ammonia or nitrite. Weekly seawater changes should be maintained even if there isn’t any sign of pollution from the nitrogen cycle tests, the minimum amount recommended is 10% of the total gallonage. Also, when corals, particularly hard corals are present a check of calcium can be made and supplemented if required. There are other supplements that could be used, these can be found at the dealers or on line but some caution is required as not all are necessarily required.

When fish are eventually introduced (compatible ones of course and not too many, one at a time over a period to allow the nitrogen cycle beasties to cope) an increase in seawater testing, that is ammonia, nitrite and nitrate is needed as fish require feeding. Many new aquarists with the best intentions overfeed and this can lead to pollution. Experience will tell when to stop feeding – fish will often still beg even though they’ve had enough and inexperience suggests more food is required.

Corals, generally speaking are not as delicate in the aquarium as appearance could suggest. Some corals are delicate and difficult to keep and others nigh on impossible. There is information on this website and online about which corals to avoid. Fortunately there are plenty of corals that are suitable and available.

On a completely unadvanced level corals can be generally divided into two types, soft and hard. The soft variety are those that sway in the seawater current and expand during the day, with colours that are often of more pastel shades. At night generally soft corals shrink until the light again returns. Hard corals could be of pastel shades too but also could be brightly coloured. Many of these shrink at night but disappear into small tubes that are all over the coral. The hard and soft coral varieties are easily distinguishable. Both can make a beautiful display but for a new reef with a beginner aquarist the soft variety are recommended.

When purchasing listen to the advice of the dealer and also research the coral on the internet ensuring it is reasonably hardy etc. The size the coral will expand to during the day is required and also the eventual size it could grow to. This growth, unlike fish is not really a problem as by the time the growth is excessive, if it ever is, the aquarist can deal with it by pruning the coral, a method usually called ‘fragging’.

Again unlike fish, corals do not have to be placed in the aquarium slowly over time. The effect of corals on seawater quality is very much lower than fish. An aquarium can take corals fairly quickly though of course never rushed. The dealer will place them in a suitable bag, the coral will have naturally shrunk. When they are home the bag(s) should be floated in the aquarium so that the temperature can equalise. An egg cup or similar of seawater from the bag can be discarded every so often, say once every 10 minutes and an egg cup of aquarium seawater put into the bag as replacement. Once the seawater has changed in the bag the coral can be temporarily gently placed in the aquarium. The light and seawater movement requirements of the coral should be known. The coral could have come on a small rock, if so great, perhaps there will be some interesting additional life. Maybe there isn’t a rock. Whatever, the coral needs to be placed in the required area ensuring that it is secure and will not fall over. A coral on a rock obviously does not need to attach itself. A coral without does and will do so over a period of time. When all corals are in place check their positions against the requirements for size, seawater flow and light and make any needed changes. Changes can be made over time but should cease as soon as possible so the corals can fully settle.

It seems very straightforward really – and it is. As always patience and research are needed so that the inmates of the reef aquarium, corals and fish are suitable and compatible. At first the aquarium will look interesting but not particularly spectacular. If the corals are happy with their placement, that is they are securely anchored and have suitable seawater flow (powerhead outlets should not directly hit a coral) and light availability is also suitable they will expand and display their beauty. The fish, if compatible and not too many of them will look terrific swimming around and over the corals. With correct maintenance as the months and years pass the reef will indeed become something very beautiful.


  1. Actually keeping corals in an aquarium is not that easy. I have learned it from my experiences. One need to get advice from those who have experiences regarding this.

  2. Hello Rip. Like all other things achieving needs knowledge so your quite correct in saying that advice from someone experienced is needed. That can be in person, on a website or a forum. The answer to keeping anything marine is the environment – get that right and your home. Some corals are quite picky and some are difficult to perhaps impossible to keep but there are plenty of others. As I’m sure you’ll agree, research is the answer.

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