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Best Plants and Algae for Refugia – Part II “Vegetable Filters”

Best Plants and Algae for Refugia – Part II “Vegetable Filters”
By: Anthony Calfo
Part I of this Series Located Here
Caulerpa was the first, popular algae used in refugiums only because the availability of alternate genera of algae in the hobby was limited. The genus Caulerpa overall though is very aggressive and sometimes toxic to fishes or invertebrates. For many aquarists, the risks of keeping large portions of Caulerpa far outweigh the benefits. Picture and Caption by Anthony Calfo

Refugiums have evolved from their earliest days in the hobby as mud and Caulerpa-filled vessels to their present multi-dimensional state. We now use refugiums for a wide range of purposes including food production, aesthetic feature, biotope displays, nutrient recycling, and “most commonly” nutrient export. In this installment, we look at this most popular and useful incarnation of the refugiums: growing safe algae for nutrient export, nicknamed the “vegetable filter.”

A nicely set-up refugium displayed here that contains liverock, some macro algae and some freshly placed Mangroves. Picture courtesy of fishgeeksrus

Considering all marine aquarists as a whole, it would be fair to say that most everybody struggles with excess nutrients in the aquarium at some point in time. This is especially true of beginners who are learning correct feeding portions and techniques.


Be sure to always strain and discard the pack juices from thawed frozen foods; they are “rocket fuel” for nuisance algae! Another flaw in many aquarium systems is a neglect of proper tuning of the protein skimmer. It is disappointing to hear some aquarists say that they only get a cup full of skimmate once every week or two (or less!). This is grossly poor skimmate production in any system with fishes or invertebrates that are fed regularly. A well-tuned skimmer can produce a dark cup of skimmate several times weekly, if not once daily. If one does not use or get such production from a skimmer, than another significant means of nutrient export is necessary. For these reasons and more (weak water change schedule, overstocking, undersized filtration, etc.), most aquariums will benefit by a refugiums that serves as a vehicle for nutrient export.

Nuisance algae like Bryopsis can be controlled or eliminated by efficient “vegetable filter” style refugiums. This is accomplished by cultivating more desirable plant or algae species to compete aggressively for the excess nutrients that fuel such pest organisms. Picture and Caption by Anthony Calfo

The premise of a “vegetable filter” is simply the cultivation of a fast-growing yet stable plant or algae species that competes for (excess) nutrients that would otherwise feed a nuisance algae growth or some other pest organism. Indeed, the competition of a healthy algae-based refugiums can limit the growth and reproduction of such notorious scourges as Aiptasia anemones and so-called Anemonia cf. majano. You might ask next, “Will such refugiums also compete with my display animals for nutrients?” The answer is, maybe. In the strictest definition of the dynamic, yes, the cultured macroalgae can compete with corals and other filter feeders for nutrients. But in practical applications, the nutrient levels in most aquaria are so very high that the “vegetable filter” cannot really even handle all of the excess nutrients available, let alone begin to actually compete with corals to their detriment for dissolved overall matter. The impact of a vegetable filter is so very easy to control too with simple pruning of the active growing algae that there is nothing to worry about. There are many macroalgae species for this purpose with great benefits and few problems or challenges.

Pictured above is a large “vegetable filter” that is thriving with Caulerpa sp. and other algaes. This refugium also incorporates a deep live bed of sand for additional biological filtration capacity. Picture courtesy of “the tiler”

So what makes a good or “best” algae for nutrient exporting refugiums? Besides being fast-growing and stable, it must also be non-aggressive. On this point, we have learned some painful lessons through the years about the genus Caulerpa. You will hear some aquarists say that Caulerpa is the best algae for refugiums, and you will find many others that say it is the worst! How can there be such a great difference of opinion? I will tell you that both are correct. In the hands of a knowledgeable aquarist that understands the needs and dangers of Caulerpa, it can be a wonderful boon. But in the hands of the unprepared and newer aquarists, it may eventually become a disaster. Although I truly admire the beauty and effectiveness of the genus Caulerpa as decor and as a vehicle for nutrient export, I rarely recommend that it be cultured in large quantities. It’s just too dangerous.

One of the biggest complaints about Caulerpa is that it is prone to sudden die-offs from stress or sexual reproduction. So many of the undesirable elements that it had absorbed into its great mass in the days and weeks prior get suddenly purged into the water during these massive, vegetative events. The shock is too great for many organisms, and aquarists have reported some catastrophic losses of display creatures after such events. Beyond any issues of toxins released, though, there is the simple fact that several kilograms of vegetable matter (the dying Caulerpa colony) are rapidly decaying in the system. The sudden proliferation of bacteria and simultaneous increase in demand for oxygen (bacterial bloom) on the decomposing colony is enough to stress or kill aquarium fishes. Ironically, this dramatic and potentially devastating disadvantage to Caulerpa is easily avoided!

There are two ways to generally reduce vegetative die-offs of Caulerpa: 1) keep it in stasis, or 2) interrupt its life cycle with strategic pruning. On the former count, “stasis” is “in layman’s terms” a state whereby the Caulerpa does not get to complete its life cycle by keeping it under constant illumination (as recommended by some advocates of the mud system refugiums keepers). In practical hobby applications, Caulerpa is one of the only algae species that this can be applied to. The majority of plants and algae must have a period of respiration (day/night photoperiods). Speaking to the latter possibility for staving off die-offs, one can simply interrupt the cycle of maturity by frequent and aggressive thinning or pruning. Thinning is best as Caulerpa fronds are single cells (the largest single-celled organisms on the planet, as I recall), to cut or break a frond itself can lead to sapping of the entire cell (releasing noxious or toxic elements), or even a complete and rapid die-off of the entire colony! A patch of Caulerpa though will naturally grow many, branching fronds which naturally break and separate on their own. Take note of the older growth and thin it out of the bunch. This is best done habitually on a weekly basis with fast growth, but no longer than monthly ideally. The natural life cycle of most species of Caulerpa falls within three to six months. The goal here is to interrupt that cycle to prevent sexual reproduction and massive die-off.

Having addressed the issue of vegetative disaster with Caulerpa, we still cannot avoid the real difficulty with the genus: namely that they are categorically more toxic than most any other common macroalgae in the hobby. They exude chemicals to inhibit the growth of other algae and even stony corals! Some creatures that repetitively graze Caulerpa have been shown to suffer (Hashimoto) as ingested toxins (caulerpin and caulerpicin) accumulate in tissues and make their way up the food web from snails to surgeonfishes, or they simply are deterred from eating the algae at all (McConnell & Hughes). For those toxins exuded in the water, we sadly cannot practically monitor or assess their “potency” as hobbyists. They are also not easily avoided and, frankly, can become a serious problem with reef keepers that do not change their carbon frequently (small portions changed weekly is best) or do regular partial water changes (25% monthly may not be enough with large colonies of Caulerpa in the system!). It is on this point (algal allelopathy) that the decision, to use Caulerpa in refugia or not, should be made. For the dedicated aquarist that is willing to set up a refugium for only one species of Caulerpa and conduct very diligent husbandry on the system (weekly chemical filtration and water exchanges), there are great benefits to be enjoyed! Caulerpa species are very beautiful, fast-growing and truly excellent vehicles for nutrient export when managed aggressively. For the average aquarist, however, (especially beginners) Caulerpa often ends up being a great challenge if not disaster. There are better algae species available that are less toxic, more stable and equally fast growing.

Green “Spaghetti algae” Chaetomorpha is one of the very best algae for nutrient export and plankton production in refugiums. It is fast-growing, non-aggressive and very stable and quite adaptable to a wide range of light. With enough water flow to keep the colony tumbling, Chaetomorpha will grow for almost any aquarist! Picture and Caption by Anthony Calfo

“Spaghetti algae” Chaetomorpha and “Ogo” Gracilaria are two of the very best macroalgae for nutrient export in “vegetable filter” style refugiums. Each has a slight advantage over the other and aquarists must weigh their options when choosing between them.


For culturing either genera is to insure strong enough waterflow in an open tank (keep no rocks or other objects on the tank bottom) so that the algae colonies grow in a constantly tumbling ball! This can be achieved or enhanced by sealing a small lip of glass or acrylic 5-8 cm wide just below the water surface (the length of the aquarium side wall) where the refugiums feed water jets into the tank. Thus, the incoming water is diffused along that slightly submerged lip and is forced to shoot across the tank surface, rather than at a deeper angle underneath. This causes a circular/rolling current of water (like an eddy in a swimming pool) to flow across the top of the tank, down the far side-wall and underneath across the bottom before coming back up again. A small starter colony of Chaetomorpha or Gracilaria will then begin to grow in a symmetrical ball and enjoy not only the benefit of evenly distributed water flow, but also the cyclic exposure of all parts of the colony to the bright light as the surface as it tumbles around!

A popular choice for refugium macroalgae among advanced aquarists in the United States is Chaetomorpha. Unlike Caulerpa, Chaetomorpha is multicellular and, as such, is inherently more stable. It can be cut and pruned aggressively with little or no fear of toxins being released or having vegetative crashes. And, like its namesake, it can also simply be thinned gently with ease like strands of spaghetti noodles. It is fast growing and makes an excellent vehicle for nutrient export! It is also highly adaptable to a very wide range of lighting [Tip: illuminate most macroalgae in refugia with at least 1 watt of light per liter of water at depths less than 30 cm]. Like most “vegetable-filter” algae species, moderate to strong water flow is necessary; keep a total water flow of at least 20X turnover of the volume of the culturing vessel. There are many other benefits to spaghetti algae, not the least of which is that it is a superb matrix for the cultivation of microcrustaceans. Aquarists with thick masses of Chaetomorpha in their refugiums get a bounty of zooplankton to feed their corals in their display, assuming the refugiums is kept without predators like fishes on the plankton. This genus is also a rather hardy shipper; trading among aquarists is strongly encouraged to distribute this useful organism in the hobby.

Gracilaria is also one of the very best algae for vegetable filters. It does require brighter light than alternative genera (2 watts per liter approximately) and appreciates very strong water movement like Chaetomorpha to keep the colony tumbling for good health and growth. With these provisions, though, it has the added benefits of great aesthetic beauty and it is quite useful for feeding herbivorous creatures. Picture and Caption by Anthony Calfo

Gracilaria can be equally effective as Chaetomorpha for nutrient export and has the added benefit of being more palatable to herbivorous fishes if (re-)cycling of the nutrients is preferred instead. In fact, the genus has been quite palatable to numerous creatures for centuries as humans can testify; it has been eaten by coastal peoples for many years and is known as the culinary delight “Ogo” to diners of Asian cuisine (it is a popularly recognized ingredient to cosmopolitan sushi diners that enjoy “seaweed salad” with their meals!). Gracilaria is a organism of great fisheries interest which generates millions of dollars in revenue in Japan and Hawaii for example. Commercial organizations culture the algae on lines or tumbling in baskets in shallow, coastal tropical waters. As a refugiums species, it has all of the same befits as Chaetomorpha (namely, multicellular and stable when pruned, fast-growing and non-toxic). It is somewhat more demanding about receiving strong water flow and really excels if kept tumbling, although Gracilaria will attach to a substrate readily. This “Ogo” is also more demanding about lighting; I recommend something close to 2 watts per liter of water in shallow refugiums (tanks less than 40 cm deep). Because of the greater demand for lighting, a tumbling colony of Gracilaria can be kept more efficiently under lower lighting, you see now.


When red Gracilaria is getting good, bright light, it will often turn orange or even slightly yellow, particularly at the fringing ends. Many aquarists associate this with poor health, but the opposite is true! We only think the color is sickly because most of us like the solid red color better. In fact, a darkening (to deep blood colored red) is actually a sign of lower light conditions and not really conducive to optimal performance in a “vegetable filter” refugiums.

In summary, there really is no one “best” species of algae at large. Rather, we must each evaluate the needs and benefits of each candidate to suit our own preferences and needs in aquarium husbandry. All three genera mentioned here can be used effectively for nutrient export. For the busy aquarist that needs minimal complications, Chaetomorpha is clearly the best algae. For enthusiasts keeping heavy populations of herbivorous fishes like Tangs/Surgeonfishes, Rabbits/Foxfaces and Angelfishes; the delightfully edible Gracilaria is the way to go. And for disciplined aquarists that are up to the challenge, the aesthetic beauty of more than forty species of decorative Caulerpa awaits them. A different flavor for every taste preference!

With kindest regards,
Anthony Calfo

Bibliography and Recommended Reading:

Anjaneyulu ASR, Prakash CVS, Raju KVS, Mallavadhani UV. 1992. “Isolation of new aromatic derivatives from a marine algal species Caulerpa racemosa.” J Natural Products 55(4): 496-499.

Faulkner, DJ. 1988. “Marine Natural Products”. Natural Products Reports. 615-616.

Hashimoto Y, Fusetani N, Nozawa K. Screening of the toxic algae on coral reefs. 569- 572.

Littlers, Diane & Mark, Bucher, Katina, and Norris, Jam, “Marine Plants and Algae of the Caribbean,” Smithsonian Books (September 1, 1992).

McConnell Oliver J., Hughes Patricia A., Targett Nancy M, Daley Joyce. 1982. “Effects of secondary metabolites from marine algae on feeding by the sea urchin, Lytechinus variegatus.” J Chemical Ecology 8(12): 1437-1453.

Meinesz, Alexandre and Simberloff, Daniel. 1999. “Killer Algae.” University of Chicago Press, Chicago. pp. 295-304.

Paul, Valerie J., and Fenical, William. 1986. “Chemical defense in tropical green algae, order Caulerpales.” Marine Ecology Progress Series 34: 157-169.