Species of Caulerpa belong to the phylum Chlorophyta, which is named for their dominant green pigment. They are some of the fastest, most prolific growers, quickly and efficiently removing nutrients from shallow coastal waters in tropical regions worldwide. In the marine aquarium there are both advantages and disadvantages to their use. Generally speaking, if used correctly, various species of the genus Caulerpa can be both ornamental and utilitarian in the aquarium. Several species however, due to their adaptability and the ease with which they can propagate themselves can be a highly invasive species, especially in reef aquaria. One such highly invasive species of Caulerpa known as taxifolia was used in the aquarium trade due to it's hardy growth and attractive appearance. In June of 2000 the introduced, non native species taxifolia was found in a coastal lagoon in Carlsbad, California, within San Diego County. An aggressive clone of this species had already proven to be highly invasive in the Mediterranean Sea, where the governments of France, Spain, Monaco, and Italy had been unable to control its spread. This first confirmed American occurrence of this invasive species in California caused considerable alarm. The resulting press coverage of the issue led to discovery of a second infestation of C. taxifolia in Huntington Harbor located in Orange County. Genetic studies determined that these two infestations were the same strain threatening the Mediterranean Sea. To date, the infested areas have been successfully eradicated, but California has placed restrictions on its import and use within the state, along with eight other species of Caulerpa. 

To control the possible infestation in non native waters the United States government issued a law making it illegal to import or transport Caulerpa taxifolia across state lines including internet sale. California passed stronger legislation making it illegal to possess, transport, transfer, release alive, import, or sell Caulerpa. taxifolia, sertularioides, mexicana, ashmeadii, scalpelliformis, racemosa, cupressoides, verticillata, and floridana. The city of San Diego took the ban one step further, banning the possession, sale, and transport of all Caulerpa species. The two species of commonly available Caulerpa species that are allowed to hobbyists in California, are prolifera and serrulata. Although I understand the need to protect the environment and our fragile coastal ecosystems, it's unlikely that any of the other eight Caulerpa species banned in the State of California, if somehow introduced, could survive the cold Pacific water temperatures. In fact most species of Caulerpa native to the State of Florida (with the exception of taxifolia and  prolifera) can't survive the cold winter temperatures of the Atlantic and Gulf of Mexico. In 2024 all Caulerpa species were banned for sale or distribution in the state of California.

They're numerous advantages to including at least some species of Caulerpa in the marine aquarium. They're not only attractive additions to the aquarium and or the refugium, but they can provide a food source for fish and invertebrates, as well as effectively removing excess nutrients. Another benefit is their ability to stabilize ph levels through photosynthesis.  There is also evidence that the addition of Caulerpa, as well as other varieties of macro algae, can aid in the reversal of many diseases often present in captive marine aquarium fish, such as fin rot and lateral line syndrome. Caulerpa remove the primary nutrients nitrate and phosphate, and in doing so decrease unwanted nuisance algae of the inorganic nutrients they need, keeping the growth of pest algae to a minimum. As the algae increases in size, the aquarist simply trims and removes any excess growth, permanently removing the sequestered nutrients from the aquarium’s system. Just like terrestrial plants, Caulerpa benefit from frequent pruning as the practice aids in removal of old tissue and encourages new growth. Hobbyists placing Caulerpa in a refugium or sump often illuminate the display tank opposite of the other which helps keep the ph stable during the algae's respiration or rest period.

Although species of Caulerpa can be beneficial, they have a few downsides. Caulerpa is a very fast grower that can quickly outgrow its environment and become a nuisance in any aquarium. In the reef aquarium, Caulerpa can effect corals by covering them and blocking available light. Some species, such as C. prolifera are believed to release a chemical that can even stunt coral growth. This is also true in their natural environment. When the amount of nutrients on a coral reef get out of balance due to fertilizer runoff and other pollutants, some opportunistic species  such as C. taxifolia, fueled by excess nutrients, can quickly take over the ecosystem smothering both native algae and corals. Another drawback is their resilient ability to spread by both sexual and asexual reproduction. Vegetative or asexual reproduction is when the algae forms a new cell from a fragment or spreads via the rhizome. This can be controlled in most aquariums by keeping the colony small and containing it to one particular area. When conditions are favorable Caulerpa will reproduce sexually, releasing gametes and tissue into the aquarium water. This phenomena is known as sporualtion and is caused by many environmental conditions such as a lack of nutrients, temperature, salinity, ph, etc. These sexual events can be detrimental to both corals and fish, sometimes releasing large amounts of nutrients and organic material back into the aquarium water. In the ocean, the effects are not harmful as the nutrients and tissue are quickly removed, however in a closed environment such an event can upset the natural balance in an aquarium if the pollutants can't be quickly absorbed by the filtration system. In larger, more stable reef aquariums dead tissue and excess nutrients are normally cleaned by both the tank inhabitants (live rock, sponges, micro algae) and or the chemical filtration system (protein skimmer, filter pads, media, etc) 

To help avoid the conditions that trigger sporualtion in the aquarium they're a few safeguards. Regular pruning of tissue is required and will help discourage Caulerpa to reproduce sexually. In addition to pruning, limiting the size of each cell is important so that biomass does not exceed the nutrient levels. If growing in a sump or refugium some hobbyists employ a continuously illuminated environment so that the Caulerpa can never enter into the phase of respiration or rest.  Although this method does work to prevent sporulation, it also effects the growth as respiration is a needed process of photosynthesis. A simpler method is to simply increase the photoperiod to allow 4-6 hours of rest, followed by continuous illumination.