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Svein Fossa
Algae Eating Animals
May 31, 1998 on #reefs

Hi everyone. I am happy to see that so many joined in to listen to what an obscure Norwegian author has to say, and that even at such an untradional hour for #reefs talks. This is my first - and possibly last ;-) - appearance on IRC. Today I want to speak on a topic which is of the utmost importance for the modern coral reef aquarium; i.e. ...

* Algae eating animals *

So - let's get on with it!!!

Coral reef aquatics are continuously becoming more widespread and popular throughout the world. Many aquarists succeed very well, while there always are those who unwind from their new hobby because they never seem to get it to function quite as they wanted it to. There is no reason to hide the fact that maintaining a modern coral reef aquarium is a demanding task for the aquarist; you will never succeed unless you really put your efforts into it. Still, it is not terribly difficult, and I fear that far too many aquarists give in rather due to a lack of patience than because of any lack of knowledge. Among the presuppositions for attaining a well functioning aquarium are, of course, a correctly assembled technical set-up, and thorough routines for the regular maintenance, such as the addition of calcium and trace elements, and cleaning of protein skimmers and mechanical filters. Even more important, however, is it that the aquarist has a certain understanding of biology and ecological principles!

The coral reef - a complicated ecosystem

The coral reef is among the most complex ecosystems we have. Only the tropical rain forest has any chance of competing with the reef in terms of abundant variety and complexity. This implies that many different species of plants and animals are mutually depending on each other, directly or indirectly. If one of these key species disappear, several other may go with it. This may lead to destroyed living conditions for one or a few species, for whole communities of interdependent species, or for the coral reef in general! Thus, the removal of key species may be life threatening for the life in the coral reef. A well known example of the important key roles of certain organisms in the ecology of the coral reefs, are the experiments which have been undertaken with the controlled removal of herbivores, or algae eating animals, from the reef. (There are a lot of animals which, to a smaller or larger extent, live from eating algae. These include fishes as well as many different invertebrates.)

Such experiments have been carried out by enclosing a limited area of the reef, by caging it in, so that none larger animals have been able to enter. When all the algae eaters thereafter have been removed, it takes only some few days until various species of algae increase enormously in numbers. Within a week or two, the whole area may be overgrown. The corals are dying; partly because they are unable to keep their surface clean and free of algae, partly due to lack of light, and partly because the coral's symbiotic algae partners, the zooxanthellae, are ousted by the other algae. The algae eating animals are, in other words, maintaining a vital, regulating function by continuously grazing on the algae so that they are prevented from taking over. But the herbivores do not remove the algae completely and altogether! There will always be a layer of short and partly nipped algae (in scientific terminology known as "Turf algae") which cover the substratum. Furthermore , there will be limited areas which the herbivores have difficulties accessing - where, hence, larger algae can grow. Exactly this fact, that the algae aren't eliminated completely, is of utmost importance! Even though the algae are rather inconspicuous in a healthy reef, they are responsible of the greater part of the production of nutrients and energy. It may, actually, be appropriate to compare their role with that of the trees of the rain forest! Besides, if the algae disappeared altogether, there would not be any more food for the algae eaters. It is the continuous growth of algae which is important. The fact that they continuously, and almost immediately, are grazed down, has no direct influence on the production. The grazing rapidly conveys the energy from the algae's production into the nutrient chain, while at the same time preventing the algae from ousting other organisms in the reef.

Algae in the aquarium

As algae are such an important component of coral reefs, it is inevitable that they also are vital to the biology of the coral reef aquarium. Various algae will always be a part of a reef aquarium, and they must (!!!) be there. Nutrients which are added to the aquarium water through food leftovers and excrements from the animals will never be completely removed by protein skimmer or other technical equipment. The algae will aid in rendering them harmless, by recirculating them in the nutrient chain through photosynthesis. In order to achieve this, there has to be growth of algae in the aquarium. However, we do not necessarily have to see much of them. A healthy, well maintained coral reef aquarium will often display an ample selection of different algae, but none should be allowed to dominate to such an extent that they become a threat to other organisms. Larger macro-algae, such as Caulerpa and kelp-like brown algae, may - as in nature - be allowed to establish themselves in lesser areas of the aquarium, where they are of little inconvenience to the sedate invertebrates.

Algae eaters in the aquarium

It should be easy to draw the conclusion that when algae eaters are a necessity for the ecology of the natural coral reef, they also have to be imperative to the aquarium environment... ... Nevertheless their absolutely vital key role is frequently underestimated by aquarists! Most will accept that some form of algae eater is required, but as soon as the first surgeonfish has been purchased, this is easily used as an excuse for spending the rest of the money on other, more enticing animals. To keep the record straight: a single algae eating animal is never enough!!! Different algae eaters will often feed on completely different forms of algae. Some prefer to eat green algae, some rather choose brown algae or calcareous algae, while yet others settle for diatoms and other microscopic species. Some herbivores eat large, fully grown algae, but the majority prefer young sprouts which still are juicy and readily digestible. Some algae eaters, such as many snails, are also experts on gnawing the tiniest algae which spread as a coating on the rocks. Certain herbivores have algae as their predominant, or even exclusive, food item. Consequently such species are capable of eating huge quantities of algae in a short time. On the other hand, several of the fish species we know as algae eaters, for example, will often choose to eat other food (dried foods, frozen foods) as well, if that is readily available. In order to achieve adequate algae control in the aquarium, it is necessary to keep several different species of algae eaters, as well as total numbers high enough to keep pace with the growth of the algae in the given aquarium.

Making the right choice

There are many animals in the trade which primarily or secondarily eat algae. There is no way I can present them all here today. Honestly I do not even know them all (and I'm sure nobudy else does so, either)! I will present some of the more important ones, though, and give an evaluation of their suitability and desirability for algae control in the aquarium. There are definitely several species of fish and invertebrates, other than those I mention, which also eat algae, but I haven chosen to emphasise those who I consider to be of the highest importance for the aquarium, while at the same time being reasonably easy to encounter in the trade. Let's start at the fishy end...


The best known, and probably also the most popular algae eating fishes are found in the family Acanthuridae, surgeonfishes. Frequently these are colourful, large fishes which are really conspicuous in an aquarium. The majority of them are also capable of eating quite a lot of algae. However, many of the surgeonfishes are unfortunately rather delicate and frequently short-lived. The transport and subsequent acclimatisation is a particularly strenuous period, where skin parasites like Cryptocarion is responsible for the death of many specimens. With a few selected exceptions, I feel that surgeonfishes are not to be recommended for the novice reef aquarist. The most hardy species are members of the genus Zebrasoma. Here are for instance the Yellow tang, Z. flavescens, which - with its intense yellow colour - is strikingly beautiful, and for long has been the most sought-after surgeonfish altogether. The Yellow tang does also remain smaller than most of its relatives. With a maximum length of 18 cm, and a reasonably tranquil way of life, it may be recommended to be kept as solitary specimens in aquaria as small as 150 litres. In larger aquaria, one may keep several specimens together, on the condition that they are introduced simultaneously and that there is an abundance of food for all of them. If not, one may risk that they will fight until bu They Yellow tang is a excellent grazer of filamentous algae, but it readily eats also most other succulent kinds of algae, such as Caulerpa. It quickly learns to accept most common substitute foods as a supplement, but algae and other kinds of vegetarian feed is always essential. Several other species of the genus Zebrasoma are similarly hardy, and choose similar food. Generally they do, however, grow much larger.

Another reasonably hardy surgeonfish is the Orange-spine Unicornfish Naso lituratus. This surgeonfish is specialised on eating various kelp-like brown algae, such as Sargassum and Dictyota. If such algae (or proper substitutes) are unavailable, the fish will starve and eventually die. Naso lituratus has an enormous appetite and will, thus, rapidly put an end to a population of brown algae in the aquarium. Luckily, it will also live well on fresh or frozen kelp from our own seas, as well as on dried Nori, the algae which is so much used in Japanese cuisine. Beware, though, that you get natural dried nori, and not one of the marinated and spiced brands! And, soak it thoroughly before use. Among the other surgeonfishes in the trade are primarily representatives of the genera Acanthurus and Ctenochaetus, and Paracanthurus hepatus. All of these seem to be more delicate, and should thus not be bought by anyone but very experienced aquarists. The genus Acanthurus contains some 37 species with, in part, highly differing choice of feed. Several species, e.g. A. achilles and A. leucosternon, eat generally the same as what the Zebrasoma-species do, while A. olivaceus and some other species are specialised on diatoms, detritus (organic debris) and small, short filamentous algae which they find on sandy bottoms and rocks. There are also species, such as Acanthurus guttatus, which gladly feed on calcareous algae in addition to green filamentous algae. Two species, A. mata and A. thompsoni, are specialised on zooplankton, and accordingly not at all algae eaters. It is highly recommended to make thorough literature studies before purchasing any Acanthurus-species. The species of Ctenochaetus are easier to sort out. The genus has a mere 6 species, which all are specialised on eating diatoms and other microscopic algae, as well as detritus. They rasp these from larger algae, corals and rocks. In most aquaria, they will thus have limited value as algae eaters, but they may be worth testing out if one is bothered with recurring problems with excessive diatom growth. j The Regal tang Paracanthurus hepatus is also frequently sold as an algae grazer, without having much value as such. Its main feed in nature is zooplankton, while algae are eaten only as a minor supplement. In an aquarium, where frozen plankton and various plankton substitutes typically are available in abundance, the Regal tang only rarely shows much interest for algae eating. All surgeonfishes grow so large that one, under no circumstances, should keep more than one specimen per 150-200 litre aquarium volume. ... and we move on to


The rabbitfishes of the family Siganidae are closely related to the surgeonfishes, and do also have several points of resemblance with these in morphology, behaviour and choice of feed. Most rabbitfishes get the bulk of their nutrients from algae. The 26 species are found in two genera, Siganus and Lo (I choose not to follow those authors who consider Lo a subgenus of Siganus). The few species of Siganus which occasionally are seen in the aquatic trade primarily feed on short filamentous algae and other choice, juicy algae while they are young. When they get older, the fishes tend to get less picky, and several of the fully grown rabbitfishes will eat practically every form of algae they might encounter. With maximum sizes in the area of 20 to 50 cm, it is a matter of course that the aquarium will have to be large. A detailed literature study is also recommendable before purchasing a specimen of an unknown species. In the genus Lo we find 4-5 species. All of them prefer filamentous algae and juicy, succulent algae which they rasp from the rocks. In the aquarium they will also accept most kinds of substitute foods. Due to the adult size of 20-25 cm, they should not be kept in aquaria smaller than some 300 litres volume. ...more algae eaters are found among the ...


Some of the best blenniid algae eaters are members of the genera Atrosalarias, Cirripectes, Ecsenius, Istiblennius, Ophioblennius, Salarias and Scartella, which all appear in the aquatic trade fairly regularly. With the exception of a single species in the genus Ecsenius (the gold-coloured E. midas which feed on zooplankton), all representatives of these genera are - as far as known - primarily herbivores. The typical choice of food is filamentous algae, which they rasp from rocks and dead corals. The majority of the species are quite small, maximum 10-15 cm, and thus first class algae grazers for smaller aquaria as well. In larger tanks, it is generally acceptable to keep several specimens together, but occasionally the fights and struggles between individuals of the same species or close relatives may be very intense, particularly if there is a lack of suitable food. The appetite for algae is normally irreproachable, and personally I have experienced that 4 specimens of Salarias fasciatus practically cleared an overgrown 3000 litre aquarium for filamentous green algae in merely a couple of weeks! On a general basis it is important to note that many blennies, under particular circumstances, might harm stony corals and certain other sessile invertebrates. Several species of Ecsenius have, for instance, been observed while eating the tissue of Acropora-corals, probably because they are able to utilise the symbiotic zooxanthellae. Scartella cristata, which is imported from the Caribbean (but also a native of the southern and eastern Mediterranean), has occasionally torn pieces of tissue from Tridacna-mussels, possibly also in order to get the zooxanthellae. Having finished with the more important herbivous fishes, I move on to invertebrates:


Despite their rather modest size, many snails are among the very best algae grazers one may find. Not least, they are valuable because they gnaw the tiniest algae, which are too small for the fishes to bother with them. Diminutive remainders of filamentous algae, various creeping, encrusting algae and microscopic species such as diatoms are all eaten by snails. The calcareous algae, which we rather will keep, are normally left alone, but the surface of these is thoroughly cleaned from epiphytic algae. The snails will, however, rarely manage to reduce a vigorous, copious population of filamentous algae. The tall algae will first have to be grazed by other animals. All aquarists who decorate their aquarium with live rock, will impromptu introduce a lot of snails as well. Most of these "stowaways" seem to be algae eaters, but occasionally some predatory species are introduced as well. One can hardly do anything about that, except observing the activity of the snails. If they are algae eaters they will soon show so through energetic grazing on rocks and aquarium glass. Several of the algae eating "stowaways" from live rock do very well in aquaria, and shows so through an enormous reproduction rate. If one has good fortune, a medium sized aquarium might soon have a population of several hundred tiny snails. "Several hundred" may sound way too much, but typically there is definitely enough food (read: algae). There is also very little risk of them dominating the aquarium from an estethical point of view. Normally they will not be conspicuous, but rather disappear between rocks and corals. In order to increase the number of algae eating snails in the aquarium, it is highly advisable to supplement with snails which are bought separately: It is primarily the more or less bowl shaped Nerita and Neritina species, plus several cone and turban shaped species which are relevant. Among the last mentioned we typically find species of Astraea (family Turbinidae) from the Caribbean, and Calliostoma, Tectus and Throchus (family Trochidae) from the Indo-Pacific. However, identification with any true certainty is difficult among snails. All of the above are excellent algae grazers for the aquarium, but the species of Nerita tend to be more delicate during transport, and more difficult to successfully acclimatise. There will, thus, frequently be a disproportionately high mortality during the first weeks after importation. Many of the Nerita-species are typical beach dwellers as well, thus having a unfortunate tendency to crawl out off the aquarium. Many of the Nerita-species are typical beach dwellers as well, thus having a unfortunate tendency to crawl out off the aquarium. Several species are known to lay eggs regularly in the aquarium. These may be seen as 1-2 mm large, white, oval spots on the decorations and glass. However, it seems that the eggs never hatch; possibly they are dependent on periodic drying out in connection with the tides. The cone and turban shaped snails are generally more hardy. Several of these are also known to spawn in aquaria, but so far very few successful rearings of the young have been reported. Apparently, for the time being, the aquarist is therefore forced to buy all the specimens he wants. As mentioned above, there is every reason to have large numbers. As a thumb rule, 1 snail per 10-15 litres aquarium volume can be recommended. The snails are very useful as they find there way into most crevices, and eat algae which rarely are touched by the fishes. ... more inverts coming up...

Hermit crabs

Among the hermit crabs we find both voracious predators and peaceful algae eaters. It used to be the larger, most colourful, and unfortunately also most predatory species, which were most frequently imported. The hairy, scarlet red Dardanus megistos is a typical example in that respect. With a length up to 15 cm and an enormous appetite, it is definitely a bad choice for the coral reef aquarium. During the last few years, it has become increasingly common to see small (normally 1-2 cm), slender species in the trade. Most of these are excellent algae grazers which concentrate on small, short algae; something in-between what the snails and the fish, like blennies, prefer to eat. Unfortunately there is no certain way to decide if a given hermit crab is an algae eater or not, unless one knows the species. There may also be predators among these small species. Seeks advice from your aquarium dealer and other aquarists who have made experiences with the current species. If you should end up with bad luck, and accidentally introduce a species which turns out to damage the invertebrates, it is fortunately not particularly difficult to catch the hermit crabs in the aquarium. ... moving on to the ...

Sea urchins

Sea urchins are known among most aquarists as excellent algae eaters. They are extremely efficient as well, but sadly uncritical regarding what they eat. When a sea urchin has crawled across the live rock in an aquarium there is rarely much left. All forms of algae, including all calcareous algae, are efficiently scraped off. In very large aquaria, or in cases where one is bothered with exceedingly strong and uncontrollable algae growth, sea urchins may be considered as algae grazers. The number of individuals must, however, be much restricted in order to avoid ending up with a completely "sterile" aquarium with gnawed down, whitish rock. As a general rule one may hint off 1 urchin per 100-200 litres aquarium volume, but then none if one has a beautiful growth of calcareous algae to take care off! Next topic is a most important one:

Introducing the algae grazers

The algae grazers should be introduced to the aquarium as the very first organisms following the live rock, together with a selection of detritus eating animals, such as serpent stars. The exact timing depends on how the maturation process in the aquarium develops. Whenever a new aquarium is started, it will nearly always go through certain, clearly defined, successive stages of development, with different algae forms dominating the different phases. After only a few days, the first diatoms settle as a smooth, brownish coating on rocks, bottom and aquarium glass. Within a few weeks these are succeeded by blue green algae (Cyanobacteria), a slimy layer with strong colours. Subsequently, the first green filamentous algae, which may dominate the aquarium fully within a month or two, appear. If the water now is free of nitrite, the time has come to introduce the first algae grazers. Remember always to give the aquarium the necessary time to stabilise. Add new and different algae eating animals as needed. Most important: Do not introduce any animals which are sensitive to algae (like most corals), or which are in need of intensive feeding (which will increase the algae growth), until you feel that you have control of the algae. Such patience is the first step on the way to the successful coral reef aquarium, which I wish you all to experience. Good luck, and thanks for listening in.

How does the efficiency of Rabbitfishes (e.g. Lo vulpinus) compare to Surgeonfishes as algae eaters, in your opinion?

It depends very much on what kind of algae growth you're trying to get rid of. However, generally speaking - I think rabbitfishes may be a bit more effective.

Which species of Ecsenius are known to harm acros?

Thats a tough question - let me say which species I personally know have harmed acros gravieri - stictus - bicolor

Do you know of an animal that can scrape thin green algae (it's hard to scrape off) from the glass in an aquarium?

Some snails may be effective, but it all depends on which species of algae you actually have.

What type will eat bubble algae?

I don't know of any snails which will do that effectively. But more generally: I'm sure that several sea urchins will tackle bubbles - but then again; they tend to be troublesome aquarium inhabitants because they destroy too much.

I have a micro reef (15 gal) a maroon clown, and striped damsel, what algae eating animals can I add?

Definitely SNAILS! Yeah - I think they would be the only algae grazers I really would recommend for your tank - unless you exchanged one of the fishes for a blenny.

Are emerald crabs or mithrax crabs effective at removing bubble algae, and are they a threat to other reef animals?

I have heard that they occationally will be effective on taking Valonia (and related genera). As for possible damage to inverts, I have no personal experience - but I fear they may be troublesome.

Any recommendations for algae ID reference books? I've got SEVERAL forms of macroalgaes and am not sure where to begin in terms of developing a "control" program for them.

There is quite a lot of algae species described (and showed in photos) in my own book The Modern Coral Reef Aquarium Vol. 1... on the other hand.. I personally get much help from Littler & al. (1989): Marine Plants of the Caribbean - which also covers several species which are found world wide

What are your feelings on these "algae kits" that are slowly coming into the marketplace and their impact on the aquarium? These kits supposedly accelerate the growth of coralline and other forms of algae.

Here again, there is a slight problem for me answering the question. The US trade appears to me to be flooded with products which are very rarely seen (if at all) in most of Europe (with the possible exception of the UK)...
... however, although I tend to be a bit sceptical to many of the new products popping up, I think it would be unfair (and unWISE) to disregard them totally.
Frequently new products have been tested well and thoroughly. The "algae kits" may very well be effective and good. But I PERSONALLY have never felt the need for such.

Any tips for getting tangs started? I've had bad luck with Zebramosa flavescens when I get them, they tend to get pinched bellies, the tank is a fish only tank, with green hair algae everywhere :(

I have seen batches of yellow tangs, where the fish obviously have suffered malnutrition.
If so happens, it will often be very difficult to get them in good shape again.
I would suggest that you always scrutinized the fish carefully for any signs of bad condition: e.g. skinny backs

Any recommendations for cyanobacteria predators?

Sorry - no, not really. However, I'm sure there are some organism out there which will do exactly that.

What kind of animals eat these "janitors" (snails, hermits)? Any recommendations on what NOT to put in with them?

Stay away from all large wrasses! There are of course also a lot of other fishes (in particular) who have molluscs and crustaceans on the menu There are also a LOT of invertebrates who actually eat molluscs and crustaceans (including several molluscs and crustaceans!) However, generally spoken: Most invertebrates which are commonly offered for sale in the trade will not be too much of a problem in this respect.

Could you please elaborate on "large wrasses"?

What we can consider as large (or not) has definitely something to do with the size of the potential prey as well. However... There are potentially problematic wrasse species in several genera, including (but not limited to): Anampses, Coris, Bodianus, Gomphosus, Thalassoma, Choerodon, and Cirrhilabrus.

I've noticed that the prevailing thought towards macroalgaes tend to be localized. Reefkeepers in my area try to eliminate macroalgaes and other areas consider them a positive part of a reef tank. For example, while valonia is typically the "Reefkeeper's Enemy", Paul Hough (GBRA) actually ADMIRED some bubble algaes in my tank while I expected him to frown on them. What is your opinion?

I have always loved macro algae - on the one condition, however, that they aren't allowed to grow uncontrolled
The problem will frequently be that -if you create the conditions which the algae need to thrive, they very easily get out of hand. But, by all means, admire the algae when you can. If (or when) they get out of control and become a pest, you'll probably think it was good but once to have loved. :-)

The Modern Coral Reef Aquarium homepage

Thanks for the talk, Svein!

© 1998

Created by liquid
Last modified 2005-02-07 05:56