Ultimate Secrets To Saltwater Fish

Idiot Guide To The Marine Aquarium

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Another kind of filamentous algae, Bryopsis, is very similar to Derbesia, except that the tips of some of the filaments have feather-like shape. The colour is dark green, often with an iridescent blue sheen. Both Derbesia and Bryopsis can occur simultaneously, growing together as tangled masses. While Derbesia is readily eaten by fish, Bryopsis is tougher to chewr and apparently not too delicious. Tangs will avoid it unless they are very hungry. Sea urchins will eat it, however. In established reef aquaria, Bryopsis usually occurs as isolated tufts on particular rocks, or on the glass. Like Enteromorpba, it commonly blooms explosively in the first several weeks after the aquarium has been set-up, causing much alarm to the aquarist not familiar with this common occurrence. The growth subsides, but unlike Enteromorpba which disappears, Bryopsis usually remains present in a few areas in the tank, unless one makes an effort to eradicate it. Removal by hand helps, but the algae grows back. Allowing turfs to grow unchecked is risky. They release spores that settle in new locations, creating new "fires" to put out. The photosynthetic Lettuce Sea Slug, Tridacbia sp., from Florida and the Caribbean, may eat Bryopsis, and is a good control. The bluish, reef-inhabiting variety of Tridacb ia crispata (which is probably a separate species) feeds on Bryopsis, while the green variety from seagrass

Above: An example of an herbivorous nudibranch, Tridachia crispata. Some forms of this nudi branch will eat Bryopsis. S.W. Michael.

Bryopsis forms thick turfs with feather-like tips. J.C. Delbeek.

Stony Corals

beds does not, usually. If the Bryopsis grows only from a particular small rock, just remove the rock from the aquarium. The rock can be held separately in the dark. When the algae has died and disappeared, zoanthid anemones and soft corals can be attached to the rock, covering the areas where the algae might have returned.


Cladopbora spp. form tangled filaments that are usually lighter green than either Derbesia or Bryopsis, and much coarser, with a consistency like polyester filter floss. Snails cannot control it when the growth is thick. They are only able to graze fine turfs. Small tangs have difficulty chewing Cladopbora, but larger tangs plow through it, devouring it quickly. They are the best control. Sea Urchins will also eat Cladopbora. Give the herbivores a head start by removing large growths by hand. The similar Cladopboropsis forms thicker, tougher filaments, and is more difficult to control.


The most common problematic hair algae is Derbesia sp. It forms dark green strands that are soft and trap a lot of detritus. It is the ' hair algae" most often encountered in reel aquariums and marine fish aquariums. It is easily controlled with tangs and herbivorous snails when the nutrient and organic load on the tank is properly managed. The tangs eat the long filaments, and the snails mow down the short turfs. Elimination of this algae can be achieved when there are sufficient numbers of herbivores. In aquariums too small for tangs, the hair algae must first be cropped by hand, and then the snails can be added to finish it off. Derbesia has an alternation of generations, with a form called the Halicystis stage that looks like a green bubble, similar to Valonia spp.

An infestation of Derbesia "hair" algae. J.C. Delbeek.

Caulerpa and Derbesia blooming in a nutrient rich aquarium. J.C. Delbeek.

Halicystis Stage DerbesiaPintura Rococo


Enteromorpha forms soft, tube-like hairs, light green in colour. It is common in the first few weeks after the tank has been established, settling all over the glass, and is typically one of the first algae to settle on new substrates in the naairal environment. In the natural environment it is an early successional species, occurring on new, bared substrates, being replaced by later growth, and a characteristic omnipresent species in high nutrient areas, especially intertidally on rocks along the shore. Enteromorpha may bloom in a reef aquarium initially, but usually disappears after several weeks, and thereafter occurs only in very small patches or not at all. It worries beginning aquarists by its sudden and dramatic appearance when it occurs, but it can easily be controlled with herbivorous fish (i.e. tangs) and it eventually subsides naturally.

Red Hair Algae

Members of the genera Ceramium, Anotrichum, and Griffithsia form red turfs, and several other algae also form either permanent red turfs, or temporary ones, as part of an alternation of generations of form. These algae are easy to control with herbivorous snails such as Astraea tectum and Turbo spp. These algae can become a problem only when there are few snails to graze them. Herbivorous fish will also crop them down eagerly, but do not eliminate them as efficiently as the snails. Apparently red turfs taste pretty good.

Cyanobacteria mat on an aquarium substrate. J.C. Delbeek.

"Slime" Algae

"Slime", "grease", "smear", or "sheet" algae are usually types of cyanobacteria, known also as "blue-green algae". They form slimy or gooey mats or sheets that are typically red to maroon, green, or black. Pale brown slime algae are usually either diatoms or dinoflagellates. However, there are brown cyanobacteria too. Slime algae mats often trap the oxygen produced during photosynthesis, the bubbles shining like little silver pearls.


Cyanobacteria are important to reef ecosystems in nature because of their ability to fix nitrogen (see chapter 2). The most common problematic types are red or maroon in color, and they can rapidly cover every surface in the aquarium under the right conditions. The right conditions in closed system aquariums involve high levels of dissolved organic compounds in the water. Removal of these dissolved organic compounds is achieved with protein skimming, as described earlier, and in chapter 5. The protein skimmer must adequately remove these organic nutrients to control cyanobacteria growth. Snails are also helpful for controlling cyanobacteria, as are herbivorous fish. We wish to emphasize that the best cure is good protein skimming. A little bit of skimming may not solve the problem, but enough skimming will do the trick. We also wish to discourage the use of antibiotics (i.e. erythromycin) or other algicides. Such remedies are not cures! They do kill the cyanobacteria, but do not prevent it from coming back. One must attack a problem at the root cause (i.e. accumulation of dissolved organic compounds, in this ease), to solve it. Cyanobacteria are

Cyanobacteria Aquarium

always present somewhere even in the most successful reef aquariums. It is only when they bloom that they become a problem. One can usually find them in clear plumbing and in the overflow chamber, both places that have high exposure to dissolved organic compounds. They are also common around the bases of soft corals, particularly Sarcophyton spp., where they apparently receive some kind of organic food. The "black band disease" of corals, in addition, involves cyanobacteria. See chapter 10 for details about that condition and how to control it.


Diatoms form golden-brown films on the glass and substrate. They are easily controlled by limiting silicate, as mentioned already, and with herbivorous snails. Diatoms are an important part of an herbivorous snail's complete diet.


Dinoflagellates occasionally bloom in reef aquaria, and they can be toxic to invertebrates and fish. They form nearly colourless to rust-brown gelatinous mats and films that trap oxygen bubbles. They can also be present in large numbers in the water column and on the surface of the water during a bloom. They coat bare surfaces so quickly that it is futile to siphon them off. The blooms can be

Dinoflagellate bloom. Note trapped oxygen bubbles. J.C. Delbeek.

persistent and maddening! From Peter Wilkens we have learned of a cure that really works. Elevating the pH to 8.4 or a little higher via kalkwasser additions effectively causes the bloom to crash within about a week, usually. See chapter 10 for a complete description and additional recommendations for control.

The dreaded "bubble algae" belong to the genus Valonia. They form clusters of spherical or oblong green bubble-like structures, actually single cells with many nuclei. They are quite attractive when few in number, like green sapphires, but that's the allure and the trick they play. They can quickly achieve epidemic proportion in the aquarium if left to their devices. They reproduce at an alarming rate, by forming from the nuclei in the cytoplasm thousands of complete miniature bubbles that brood within each "parent" bubble. When the old cell wall tears, these baby bubbles are released, and they settle all over the substrate. The problem is that they encroach on living coral tissue and cause it to recede (see chapter 10). Control after a bloom has already occurred is difficult. Prevention is best. Remove individual bubbles by grabbing them

Valonia sp. algae crowding out zoanthid polyps. J.C. Delbeek.

with a needle-nose pliers or small scissors. Twist and yank them off, and remove them from the aquarium. Don't worry too much about popping them (which does release the tiny brooded bubbles if the algae is in the reproductive stage), but avoid it if you can. If the bubbles are already in plague proportions, disassemble the reef structure and pull them off outside the aquarium. After the rock is reassembled in the aquarium, if the surfaces are covered by anemones, soft corals, and stony corals, there will be less available surface for Valonia to grow, and any new bubbles can be seen and easily removed as a general maintenance procedure every few weeks. In time, if purple coralline algae are encouraged to grow vigorously and coat the bare surfaces of the rocks, Valonia will become rare and maintenance will be less critical. Valonia seldom bloom in aquariums with heavy growth of coralline algae.


There are numerous herbivorous fish that can be utilized for control of algae in the reef aquarium.

The most efficient herbivorous fish are tangs, particularly members of the genus Zebrasoma that have a long snout. The Yellow tang from Hawaii, Zebrasoma flavescens is the most readily available. The Sailfin tangs, Zebrasoma veliferum from the Pacific, and Z. desjardinii from the Indo-Pacific and Red Sea are also excellent grazers, as are the Scopas tang, Z. scopas, and the Red Sea Purple tang Z. xanthurus. All of these eat filamentous algae and are capable of grazing even tough algae, like Cladopbora and Cladopboropsis.

The Red Sea Purple Tang, Zebrasoma xanthurus. J.C. Delbeek.

Large specimens of Zebrasoma desjardinii have been reported to feed on l/a/0/7/a and other problematic algae. J.C. Delbeek.

Stony CoralForeign Legion RanksYellow Tang Lip Problems

The Yellow Tang, Zebrasoma flavescens, is an excellent herbivore in the reef aquarium. J.C. Delbeek.

Tangs of the genus Ctenochaetus, such as this Chevron tang, Ctenochaetus hawaiiensis, are excellent grazers of microalgae, but beware that they are very prone to Amyloodinium. J.C. Delbeek.

Ctenocbaetus tangs, such as the Kole, C. striatus, and the Chevron tang, C. hawaiiensis, have broad, fat lips that they bang and flap across the rocks constantly, scraping up short algal turfs, slimy algal mats, and detritus. They effectively polish the rock, like snails, with their brush-like teeth.

Beware that all tangs are especially prone to fish diseases such as "Ich" and Amyloodinium. Yellow7 tangs are prone to "black spot" disease. See Moe 1992, Blasiola, 1992, and computer programs listed in appendix C for details about treatment of fish disease.

Some types of blennies are excellent herbivores. Salarias fasciatus, from the Pacific, and the Red-lipped Blenny, Ophioblennius atlanticus, from Florida and the Caribbean, have lips very similar to the Ctenocbaetus tangs, and they eat in the same manner, though it is a bit more comical to watch one of these graze as it seems to bang its entire head against the rocks. Red-lipped Blennies are very territorial and nippy, however, and they can be bothersome especially in small aquariums. Not all blennies eat algae. Some are carnivorous, and others that have the same kind of brush-lipped face as the algae eaters actually feed exclusively on coral polyps (i.e. Exallias brevis). Be careful with blennies! (see also chapter 10, poor expansion).

A lew types of gobies also eat algae. Members of the genus Amblygobius, in addition to sifting sand, graze filamentous algae from the rocks. Amblygobius rainfordi and A. phalaena may starve in an aquarium that is completely devoid of filamentous algae. These gobies are quite shy, and do not do w7ell in an aquarium with aggressive tankmates (see Delbeek and Michael. 1993).

Amblygobius rainfordi are timid fish that feed on filamentous red and green algae. J.C. Delbeek.

Amblygobius Rainfordi

Pygmy angel fish of the genus Centropyge make attractive additions to any aquarium, and they are good herbivores as well. Unfortunately, they do not always restrict their diet to algae. They may begin to feed on corals and clams suddenly, and without mercy, much to the aquarist's chagrin. In our experience, C. argi seldom bothers corals. Popular species such as C. loriculus, the Flame angel, and C. acanthops, the African Flameback angel, have proven unpredictable. Sometimes they bother corals and clams, sometimes they don't, and sometimes they don't bother them for years, but change their habit suddenly.

We should warn you that all of these herbivorous fish are inclined to mistake soft corals and sometimes even stony corals for algae on occasion, nibbling on the polyps, but usually do no permanent harm. The Centropyge angels, however, often develop an appetite for coral and tridacnid clams, so their inclusion in a reef aquarium is a little risky (see chapter 10).

Herbivorous Snails

Several species of herbivorous snails are available to aquarists, from the genera Astraea, Turbo, Trochus, Nerita, Ceritbium and Calliostoma. Other herbivorous molluscs include limpets, chitons, and abalones. Some herbivorous snails grow large and have an undesirable attribute of knocking things over in their search for food. Some don't live very long, or are easily preyed upon. Ideally the rate of attrition of the snails is low, and only a few will need to be added per year, but sometimes fish and other predators will cause heavy7 losses. A few types readily reproduce in small aquariums, and all types do reproduce successfully in really large

The African Flameback angel, Centropyge acanthops. J.C. Delbeek.

Herbivorous Snails

aquariums. Herbivorous snails are a true blessing from the reefs. They make the management of algae really simple, and they are more efficient than fish at removing algae from crevices in the rocks. Still, the best way to manage the growth of algae with herbivores is to use both fish and snails. The fish graze the longer growths, and the snails polish off the fine turfs.

Astraea tectum and Astraea caelata (Trochacea) are excellent snails from Florida and the Caribbean. They are long-lived, and do not grow very large, so they aren't too bad about knocking things over. Use about one snail per gallon for the best results in small reef aquariums. In large aquariums, 2000 L (500 gal.) or more, fewer snails may be needed as the surface of the rock exposed to light does not increase as dramatically as the water volume.

The larger snail is Turbo fluctuosus. The smaller snail riding on its back is Astraea (Lithopoma) tectum. Small Trochussp. from the Indo-Pacific look like Astraea. They grow very large, however. J. Sprung.

Bryopsis Algae Control

Cerith snails shown here are common on live rock trom the

Caribbean. They feed on diatoms primarily, but larger ones feed on some turt algae. Some species live on sand intertidally, migrating with the tide. J. Sprung.

Nerites are common intertidal snails that feed on diatoms, cyanobacteria, and turf algae. Some species must crawl out of the water to breathe, and are unsuitable for aquariums without lids. They wander too far. The two attractive species shown here are good for reef aquariums. J. Sprung,

Ctenochaetus Hawaiiensis

Turbo species (Trochacea) are very popular for algae control. They grow fairly large, about 8cm (3 in.), and have a voracious appetite, so fewer are needed. About two snails per 60 liters (15 gallons) is sufficient. Because of their large size they are less desirable in small reef aquariums, less than 120 liters (30 gallons), since they can knock things over. Some species aren't hardy in captivity, possibly as a result of temperature or shortage of food.

Trocbus species (Trochacea) are similar to Astraea in appearance. They are also excellent consumers of filamentous algae! Trocbus niloticus, which has been depleted from some locations by Pacific islanders who eat them and use the shells for a variety of ornamental purposes, is now being farm raised by the MMDC in Palau. Initially reared as a means of algae control in grow-out

Stony Stonewart Alage Killer

raceways for tridacnid clams, the snails are now being sold to the aquarium industry. These are the only farm raised snails available. All others are taken from rocky coasts and seagrass meadows. In the future it is likely that other Trochacea such as Turbo and Astraea will be farm raised for aquarists. Although Trochus niloticus is a fine algae eater, it does grow very large, ultimately too large for small aquariums. Fortunately small juveniles are offered for sale. These snails are long lived.

Limpets, which look like a Chinese hat, also are introduced with the rocks, and some species such as Diodora, the keyhole limpet, will reproduce in the aquarium. They are all good herbivores, but sometimes they will eat both soft and stony corals (see chapter 10).

Nerite snails, Neritaspp., are usually restricted to the intertidal region, while most other herbivorous snails occur in both intertidal and subtidal regions. Some nerites are unsuitable for aquariums since they crawl out, leave a slimy trail across the floor, and go "crunch" underfoot. Some nerites do adapt well to aquariums, occasionally going above the wrater line, but usually remaining in the aquarium and eating algae. Nerites are best employed as a supplemental variety of herbivorous snail, in addition to the primary ones such as Turbo, Trochus, and Astraea.

Another supplemental variety of snail, Ceritbium litteratum is usually introduced to the aquarium with live rock. It remains quite small and is long-lived, but it is not very efficient at consuming filamentous algae, preferring diatoms and cyanobacteria.

A small herbivorous snail, Stomatella varia, that looks like an abalone, is common on live rocks from Indonesia. These are actually related to Turbo and Astraea, family Trochacea, despite their appearance. They readily reproduce in the aquarium, like limpets. They do not harm corals, remain small, and are excellent, voracious herbivores. Though they cannot tackle dense turfs of filamentous algae, they are good for controlling diatoms.

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  • Ollie
    What causes cyanobacteria in reef tank?
    3 years ago

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