In marine environments, the organisms that are most commonly referred to as “seaweed” are, in actuality, not true plants but rather marine algae. Found in both single celled and multicelled forms, marine algae are distinguished from plants by their absence of roots, stems, and leaves. They do, however, possess the equal ecological equivalents of these structures, although such may function somewhat differently. For example, the holdfast of a marine algae, superficially similar to a plant's roots, does not absorb nutrients but rather serves only to anchor the organism. Marine algae take in nutrients through leaflike structures that vary widely in shape from species to species.

One of the few true plants to be found in marine environments is eelgrass. Eelgrass beds are favored habitats for a wide variety of interesting creatures, but the plant itself has not often been established in aquariums. The aquarist with an interest in plants would do well to experiment with growing eelgrass in captivity.

Perhaps the first algae to establish itself in your marine aquarium will not be deliberately planted but rather will arrive attached to rocks, corals, or sessile invertebrates. Most commonly seen are various species of green algae that form a fuzzy coating on aquarium surfaces. These types greatly improve the appearance of artificial coral and plastic plants by lending them a realistic look and are an excellent food source for a variety of small fish and invertebrates. As described above, harmful species should be removed, lest they out-compete more desirable organisms.

Various species of Caulerpa are among the most readily available and resilient of the marine algae. This genus's members take on a variety of forms, depending upon the species. Some produced rounded leaves, while others grow blades that may be straight or fernlike in appearance. Most species can develop quite dense stands after a time and are useful as animal shelters and sight barriers.

Perhaps the most common species is Caulerpa prolifera, a native of warm waters extending southward from Florida. It is commercially cultivated in Europe and is sold under a variety of common names. Other related species occur throughout Southeast Asia and in the Mediterranean Sea.

Caulerpa spp. spread via the use of runners, or rhizomes, that grow out over various substrates. Several species, such as C. prolifera and C. asheadii may grow very quickly and can overwhelm other algae and sessile invertebrates. To avoid shocking the algae, it is best to stay ahead of such situations by pruning in small amounts at a time so that a major cutting is not necessary.

This aptly named marine algae belongs to a group known as the “calcareous algae.” The leaflike structures of the mermaid's shaving brush and other calcareous species absorb calcium from the water. Such algae are, therefore, fairly rigid and stand up well to the attentions of herbivorous aquarium animals.

Sea cactus lack spines, but in their round, clustered appearance otherwise resemble their terrestrial namesakes. A calcareous algae, they are quite sturdy and therefore suitable for planting in aquariums that house large, active fish and invertebrates. The sea cactus is a warm water species and requires a fairly high pH but is otherwise undemanding and thrives well in marine aquariums. A number of other species of green algae, most often of the genus Halimeda, take the form of flat, platelike structures and are often sold in the trade under names such as “cactus algae.”

This algae forms a cluster of pale green, yellow-centered cups and is quite beautiful to look at. It is, however, very delicate and cannot withstand the nibbling of herbivorous fish or being trampled by vigorous invertebrates. The mermaid's cup requires placid waters and is best suited to aquariums housing sessile invertebrates and small, peaceful fish. It is a fairly slow grower and can be quickly overwhelmed by more vigorous species of algae.

Members of this genus have been, in recent times, appearing more often in the pet trade. Most are at home in the warm waters of the Caribbean Sea and, being calcareous, are quite sturdy and fairly unpalatable to most aquatic creatures. Most Codiacea grow slowly but will do well if provided with intense light and suitably high pH levels. Various species may be red-dish or brown in coloration.

While most of the commercially available species of marine algae are green, bright red or maroon specimens are occasionally offered for sale. Where this is legal, these may also be collected and established in the aquarium. Many types grow attached by thick holdfasts to small stones, and form dense thickets that are excellent shelters for many marine creatures. In the aquarium, their coloration offers a welcome bit of contrast to the more typical marine algae and corals.