Still Impossible After All These Years: Keeping Moorish Idols

Author: Kieron Dodds

The Moorish idol is a beautiful fish that tempts many aquarists to try keeping it in home aquariums, but as the author demonstrates, it’s still an impossible task

The End

Beginning a story at its conclusion is usually reserved for television crime dramas or Hollywood thrillers. I can’t promise you anything as nail-biting as that, but I can promise you one thing: if you are a conscientious aquarist, you will get chills at the end of this article more real than any that can be gotten at the end of any fictional story.

The Moorish idol Zanclus cornutus, or in some references Z. canescens, is still to be considered a species heartbreakingly impossible to keep, even after all of the advances in marine fishkeeping in the last three decades. So I begin my report here with its end—the death of all eight individuals of this incredibly beautiful, graceful, and majestic species that I tried to keep.

Hundreds, perhaps thousands, of individual Moorish idols are removed from the reefs each season, but it’s doubtful that more than a handful of specimens will survive in captivity for more than a few months, with most succumbing in transit or in hobbyist tanks within a few days or weeks. To be blunt, the hobby as a whole continues to remain completely clueless as to how to properly capture, transport, and house this gorgeous species in a way that ensures even a moderately acceptable chance of survival.

Not an Endorsement

Make no mistake, this article is not intended to be a guide as to how to obtain and keep Moorish idols successfully. It is a recommendation against keeping this fish, period. Based on the experience of the mere handful of aquarists who have kept specimens alive for more than a few months, chances are that any who try to keep Moorish idols will fail—even if the procedures herein are properly carried out.

Failure Almost Assured

Almost every hobbyist who will try to keep Zanclus cornutus for any reasonable amount of time will fail, and fail miserably. Does that sound harsh? It should. It’s a harsh reality. Then why write an article that gives husbandry information for this species? Isn’t that hypocritical? When I discussed this article initially with TFH editors and other aquarists, it became painfully evident that, to some extent, it does encourage some measure of irresponsibility to offer information about Moorish idol care.

Pioneering efforts in this hobby, however, are risky, and there is a very fine line between taking necessary risks and irresponsibility. Some of the key differences are research, resources, and willingness to do what is required to succeed. Ultimately, however, there is no justification for failure, just as there is no justification for success. The responsible thing to do is to leave Moorish idols in the wild.

My Experiences

A perennial on the list of favorites for many aquarists, Zanclus cornutus has broken the hearts, minds, and spirits of almost every aquarist who has tried to keep it. I kept this in mind as I planned the stocking list for my new reef tank almost three years ago. The Z. cornutus specimens I obtained were two of the initial entries into my newly set-up 450-gallon tank, and they did not fare well at all. With almost 30 years of experience in this hobby, I did no better than would a beginner aquarist.

Although the pair of Moorish idols I obtained ate well and seemed to be doing okay, an unexpected temperature spike to almost 90°F (something almost all reef fish can easily withstand), followed by a nitrite spike about a week after their initial acquisition, rapidly did them in. Both were dead within three days of the initial temperature increase.

Tools for Success

I thought about it for quite some time and came to the conclusion that, although Moorish idols were a risk, if I waited for stability to be achieved, I was one of very few people who did have the tools necessary for success. But my initial experience with them had me discouraged. I looked at that failure as a judgment upon my abilities as an aquarist, and not as the huge mistake that it was. The Moorish idols I obtained were, after all, feeding, gaining weight, fitting in, and looking great up until the point of the temperature and nitrite spikes.

What brought me to the assumption that this early introduction was probably just a stupid mistake was the next acquisition I made for the tank some two months later, a powder blue tang Acanthurus leucosternon. Regarded by many as one of the most difficult acanthurids, this fish did beautifully until a two-day power outage claimed it, along with half of the other fish in the tank. However, that came months later.

Learning from Mistakes

Before that unfortunate power outage, I was observing some very difficult species doing exceptionally well in this tank. This is more along the lines of my normal experience with difficult species. So I thought that, while I certainly couldn’t stop kicking myself for making a stupid mistake, I could move on and try to learn from that mistake. That concluded the decision-making process that brought me to try Moorish idols a second time.

I began researching heavily, and I followed ongoing conversations between aquarists that were experiencing some success. I introduced a single specimen in early 2006, and after it showed signs of thriving I added another individual some weeks later. Based on the amount of abuse this second fish initially took, I would not recommend introducing a second Moorish idol into a tank. Both of these fish did exceptionally well up until the power outage occurred some seven months later. I blamed the power failure for their deaths, but what I did not consider was that there were many fish that did survive the outage, and they continue to thrive even to this day.


Subsequent attempts to keep this species have been met with incredibly miserable failures measured in weeks, not months. Some can be attributed to failure to eat, others to the other fish in the tank being too aggressive for the newly introduced fish. One thing is almost always true: if a Moorish idol stops feeding, it probably will never feed again.

The Experiences of Others

True success stories with this species are few and far between, but those with positive experiences are usually more than willing to talk about them. Sadly, however, most people are not very willing to share the particulars of their failures. I found that people reporting success or failure were at about a one-to-one ratio. The initial “success” stories are very misleading, however. My secondary inquiries indicate that the actual success rate was much, much lower, probably less than 10 percent beyond the first year. Once again, the evidence indicates that simply acquiring this species is virtually a guarantee of failure.

Common Tales

There are many commonalities in the practices of those who have had success (however temporary), and it is these practices that the aquarist who has already purchased a Moorish idol should focus on. With this very delicate species, even a minute deviation from such recommendations can tip the scales even more in favor of failure. Skimping on a single item, or taking chances, even on one small point, often results in disastrous consequences. But no matter what is done, with our current understanding, the chance of failure is almost certain.

Again, the husbandry information that follows is not a recommendation, but rather should be seen as a minimum requirement, based on what a few have achieved. While these achievements almost always eventually resulted in failure, at least longer lengths of time elapsed before those failures occurred.

One Success Story

Pablo Tepoot is, perhaps, the single individual who has had the most success with this species, and even he reports (pers. comm., Tepoot, 2006) no better than a one-in-seven success ratio with this species. Please keep that equation in mind when you see a tank full of Moorish idols on display. If they have a dozen on display, it is likely that 72 or more other specimens died.

If you decide to try keeping a Moorish idol, please ask yourself seriously if those numbers are acceptable. For me, they are not, and I will not be including this fish in any of my tanks at any time in the near future. The following information is for those who already have a Moorish idol, and to give a fighting—if feeble—chance to any specimens acquired despite these warnings.

Tank Size and Environment

Tank Parameters

It cannot be overstated that one of the most important factors in keeping Moorish idols successfully is the size of the aquarium in which they are housed. A barely acceptable minimum would be a tank approaching 200 gallons in volume—the absolute minimum for keeping post-larval juveniles. A larger aquarium, preferably twice as large, is a better minimum target volume. It is hard to believe just how much room these fish actually require.

They bolt and achieve top speed in the blink of an eye, and can stop on a dime and flit back in the same direction they just came from. They will swim sideways across a sandbed at almost full speed to evade pursuit or look for morsels. Perhaps the most impressive feat—something that most aquarists will never see in smaller tanks—is that they will actually swim loops, like a fighter pilot, coming down behind and facing the tail end of a fish that previously was pursuing them. All of these top-speed acrobatics and extended swimming require not only a long, tall tank, but one that also has enough open bottom, requiring a deep (front to back) tank as well.


Not only does the tank need to be large, but the accommodations within, in terms of live rock and reef structure, are also very important. Moorish idols are grazers in their wild state, and according to reports, they graze mostly on sponge. This means that there must be sufficient reef structure for the fish to graze upon.

The structure itself should be very open, allowing plenty of room for a full-grown Moorish idol to easily swim through in several places. But with an aquarium as small as 500 gallons, a single Moorish idol will wipe the tank clean of any sponge it finds palatable within a day or two. Even sponge grown in a refugium and fed to the fish in the main tank will quickly disappear.

There is, nevertheless, an observed advantage in having plenty of mature live rock for the fish to pick over: the food items it finds can, in part, make up for deficiencies in the captive diet. It’s a very small part, however, and providing a naturally renewable food source within the tank is impossible.

Water Movement

Moorish idols are reef fishes and as such are accustomed to a fair amount of turbulence. For such free-swimming reef species, sluggish water can make them seem sluggish, and it actually places more stress upon them than if there were a lot more movement. Also, a very high flow rate will aid in oxygenation of the water, and those who have had any success with the species have reported that highly oxygenated water is a must.

The total flow rate should not be less than 20 times the tank volume per hour. Besides total turnover, it is important to provide enough movement to eliminate dead spots that can become nutrient and bacterial sinks, since they lead to a decrease in water quality.

Water Quality

Dead spots within the aquarium can lead to detritus accumulation in small areas where any detritivorous organisms cannot keep up. What this amounts to is rising bacterial levels—and nitrate and phosphate levels can soar, often leaving the aquarist perplexed as to how or why. Besides nutrient levels having a negative effect on some forms of reefkeeping, high bacteria concentration has also been reported to be detrimental to some fishes—mostly to those that are cartilaginous, but also to Moorish idols.

It is common practice in many local fish stores to keep the salinity low and treat with preventative medications, which are often copper based. This is not an inherently bad thing for most species, but for the long-term maintenance of more delicate species, including the Moorish idol, it often is. It is in a fish store’s best interest, however, to provide the best short-term care for the highest number of species. In this sense, using preventative measures is often better than providing long-term conditions suitable for more delicate species. Moorish idols kept this way will color down fairly rapidly, and the longer the fish are kept under these conditions, the less likely they are to thrive.

They also result in the Moorish idols developing ulcerations, which may develop anywhere on the body, looking like chunks of flesh have fallen or been removed from the fish. Very rarely will there be any reddening or bleeding in the area of the ulceration. Fins will also rip and tear, regardless of display tank size or other species housed within it. The individual Moorish idol specimen, even if it is eating, will be less and less likely to continue to do so as days and weeks pass. So add perfect water parameters to the list of minimum requirements for keeping a Moorish idol.

The Aquarium as a Whole

Knowledgeable aquarists will look at the above recommendations and immediately intuit that nothing less than a well-established and well-maintained large aquarium will suit a Moorish idol. These fish should never be placed into a new tank, with “new” meaning any aquarium that has not been completely stable for at least a year (not simply set up for a year or more).

The importance of the stability of the environment cannot be overstated. Virtually all aquarists reporting Zanclus longevities of more than a few months are keeping their aquariums steady and as close to natural-sea-water parameters as possible.


Moorish idols are not wallflowers at all; they can hold their own against most other species, including (as I’ve seen evidence of) already established, belligerent acanthurids. However, in the interest of eliminating as much stress upon the fish as possible, tankmates should be chosen wisely. Where fish are concerned, this means that competition should be kept to a minimum.

Species to Avoid

With that in mind, the larger pomacanthids are probably among the worst tankmate choices. Acanthurids are also competitors and can be quite pugnacious as well.

Good Tankmates

Other competing species are generally not as much of a concern. It would be safe to say that, when it comes to behavior and competition, the Moorish idol should be one of the first species added to any aquarium. But this is not a solution and is not advisable. The aquarist cannot test the stability of the system by allowing it to run fallow of fish for a year or more. Good choices for introduction prior to the Moorish idols would be planktivores like Chromis spp., carnivorous grazers such as wrasses, and cave-dwellers such as more peaceful dottybacks.


Motile invertebrates are generally not a problem with Moorish idols. In this respect, these fish can be deemed reef safe. However, in the absence of naturally available food types, they have been known to pick at corals and clams. The sessile invertebrates most susceptible to this picking seem to be smaller clams (under 4 to 6 inches), any fleshy large-polyped scleractinian corals, and larger feather-duster worms. Small-polyp scleractinians, as well as large-polyp scleractinian corals with potent stings like Euphyllia spp. and Galaxea spp., seem to be perfectly safe from predation by Moorish idols.

Diet and Feeding

One of the greatest hurdles (if not the greatest) in keeping Moorish idols is feeding them. Substitutes for their reported natural sponge diet always seem to eventually fail in most cases. What is usually described is a fish that feeds fairly well on a particular diet (usually herbivore based), regains some weight, keeps that weight on for a fairly long period of time, then starts to slowly waste away.

Sponge Matter

A diet composed of a large amount of sponge matter would be ideal. Unfortunately even those commercial diets that contain sponge matter, like those for angelfish, list their primary ingredients as being crustacean or mollusk based, and/or vegetable based. Interestingly, those aquarists reporting the greatest success with Moorish idols offer living sponge—whether temperate or tropical—at least as a small portion of the diet.


Procuring substitute foods that contain at least some sponge matter is only a small portion of the problem, however. The real challenge seems to be in getting the fish to accept any food other than sponge—and sometimes even sponge! Some have reported great success with pellet foods developed for finicky feeders, even offering such foods in dishes to maintain concentration for the Moorish idols. Others who have tried such foods have reported little to no success. Mashing gelatin-based frozen foods into rocks or clam shells is another trick that sometimes works. While nori and other vegetable-based products are often recommended for species that are primarily spongivores, I am a little leery of such recommendations.

There is no real evidence to indicate that macroalgae are a viable nutritional substitute for sponge. Still, getting the Moorish idol to eat something is extremely important. The more it eats, the more likely you will be to get it to live long enough to subsist on a wide variety of offered foods, and the more time you buy until you can offer it sponge as a primary diet. To this end, mysid shrimp, bloodworms, nori sheets, halved freshwater clams, and other highly palatable foods should be offered regularly.

Finding Sponges

There are quite a number of Moorish-idol keepers who are relying on live sponge as a good portion of their fish’s diet. Some are collecting local sponges—Pacific or Atlantic, temperate or tropical—and offering them with great success. (If you plan to collect sponges or any other marine life, of course, make sure you know and follow all local laws and regulations.) Others are buying live sponges from their local fish store and offering those with equal success. Because some sponges may contain toxins, and some collected close to shore may contain harmful pollutants, this may not be an advisable solution, but it does seem to be working for a number of people.

Specimens and Behavior

Other than feeding, acquiring the right specimen is probably the most difficult part of keeping the Moorish idol. Before selecting specimens the aquarist should know that Moorish idols are most commonly found in pairs, although they will school naturally. Whether or not these are always male/female pairs seems unclear, but some indications point to this as being the case. With many marine species, you can simply obtain two juveniles to acquire a pair since one will eventually change sex, but it is unknown if this is the case with the Moorish idol. Whatever the case may be, there does seem to be some indication that they do better if they are kept in pairs, harems, or schools, rather than singly.

Finding Specimens

So the first hurdle is in finding more than one specimen at the same time. If it is even possible, the specimens still all need to be suitable. But what constitutes a suitable specimen? Other than normal healthy appearance issues—the fish should not be emaciated, not have any bodily damage, and should not show any symptoms of any disease—it can be tricky to identify an individual that is acceptable. Some fin damage is usually inevitable, especially to the dorsal streamer, and it is usually not problematic. Washed-out coloration is a common occurrence if the store keeps the specific gravity of the display tank a bit on the low side, or if they maintain treatment levels of medications, especially copper.

Never purchase a Moorish idol that is not already feeding. It would be better if it was feeding with gusto, but this would be an extremely rare individual. The size of the specimen also seems to play a pretty big role in success or failure. The smaller the individual, the younger it is, and hence the greater chance you will have success with it. Individuals between the 3- and 4-inch mark seem to do somewhat better than those larger or smaller. Any individual that does not appear to be perfect should be automatically disqualified from purchase.

I chose eight specimens, but I turned away eighteen other individuals. From what I have experienced, there are easily twice as many specimens that are doomed before they even leave a fish store as there are fish that reach those stores in a healthy condition.


Contrary to most recommendations, any specimens purchased should be quickly removed from situations that are not as near perfect as possible. Getting a specimen into an appropriate setting as soon as possible seems to go a long way toward long-term success.

This also holds true for quarantining specimens. If a quarantine tank that suits the Moorish idol’s long-term care is not available, then it may be better to forego quarantine. There are many, many reports of otherwise perfect individuals dying in quarantine tanks within days of acquisition. Conditions in quarantine should match the intended long-term aquarium as closely as possible. Moorish idols do not transport well, and each acclimation that is done before they reach their final home seems to take more out of them.

Irresponsible Is Irresponsible

The Moorish idol is one of the species that is most asked about, but it is usually never given a chance by those responding to such queries, and rightfully so. The recommendations from acknowledged experts all follow the same thoughts: the Moorish idol is better off left in the ocean. However, there will always be experienced aquarists willing to accept the challenge, and others who insist upon keeping the Moorish idol, regardless of warnings.

After reading this article, perhaps some readers who may have otherwise failed at keeping this species will now succeed. But hopefully, most will not even attempt to keep this species at all. When the fish have already been obtained (or in very rare instances, when they are acquired by an expert attempting to uncover the secrets to successfully keeping them), this article can serve as a concise and compiled reference to which the aquarist can refer.

While it is the main intent of this article to discourage the acquisition of Moorish idols, it is also to acknowledge that there will always be a market for the species, and that reference material for their care is sorely lacking. It is hoped that you, the reader, will do the responsible thing and leave Moorish idols in the wild; if not, at least have enough information available for this most demanding and impossible of species.

The Body Count

And here we conclude, exactly where we began, just like the wrap-up scene in a crime drama, with a pile of bodies. This article was over two years in the writing; I was gathering data and constantly researching anecdotal reports from hundreds of individual Moorish-idol keepers. Two years ago, I approached this species with bright eyes, full of wonder and hope. That was the beginning.

Today, I still look at them with bright eyes, and they still fill me with wonder. But I, like many before me, have had my hopes dashed by Zanclus cornutus. And here, the analogy to a crime drama is not so far off. Perhaps, as conscientious aquarists, we should close our eyes now and imagine the body count playing in this horrible end scene. In my home, eight bodies lined up, and in countless other homes many, many more…bodies upon bodies....

Is It Worth It?

After reading this, knowing that the strict keeping conditions contained herein were still met with failure eight times and met with success zero times, will you still purchase that Moorish idol? It is my sincerest hope that you will not.

Are you ready for the total body count, one I have been tallying for the nearly three years it took to conceive, plan, and write this article? It’s 382 bodies. This table details how long the various fish I had been keeping tabs on survived in hobbyist aquariums:

No Survivors

Three hundred eighty-two bodies and not one single living individual made it through the time it took me to finish this article, the last having been reported dead as this article was going to press. Zero made it.

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