The wracks are brown seaweeds related to the kelps, such as konbu and wakame. Brown seaweeds are actually the dominant forms in colder oceans; some of the kelps actually form underwater forests, whereas their kin the wracks clothe the intertidal shore. Both clades are much less dominant at low latitudes, with the exception of free-floating, pelagic wtacks forming the wonderful Sargasso Sea in the doldrums - the vast, tropical Atlantic region of calm, almost windless waters and still, blue sea that can still trap sailing vessels.
Two free floating Sargassum sp. exist there in vast masses, inspiring great nautical folklore. The Sargasso Sea was first known to the Carthaginians, according to the Romans; it is also possible that knowledge of the Sargasso Sea entered Plato's syncretic account of Atlantis. Incidentally the word 'sargasso' entered English via Portuguese; the Iberian mariners named the seaweed after a familiar land plant. (Needless to say that wracks are unrelated to any land plants.) In English the Sargassum genus can be called gulfweeds or sargassos.
Although brown seaweeds are rarely traded deliberately as aquarium plants, they do appear commonly enough as accidental 'hitch hikers'. Although Sargassum sp. are most famous for their floating Atlantic communities, they can also be important as attached plants near coral reefs; this is because there is more than one species of Sargassum. One of the attached species is the white-veined sargasso, Sargassum hystrix, which does not contribute to the floating masses that terrified sailors, and wash adore on the eastern US shore. Is is however traded as an ornamental seaweed.
S. hystrix inhabits rocky coastal areas where it grows attached on outer reef and fire reef slopes on the Western Atlantic and Caribbean. It grows up to 40 centimeters or 16 inches tall, unlike some Sargassum sp. that grow truly vast. So it is possible to grow a manageable sargasso in one's reef tank, if one can source one. Sargassum sp. contain toxins that hinder coral growth, however this is in effect only when corals are in physical contact with sargasso.
S. hystrix and the other anchored sargassos, possess sturdy stems and, despite their relationship to the floating sargassos, they possess firm holdfasts. In the wild and in the aquarium, they handle more flow than do those seaweeds which are more delicate in this regard. Wild S. hystrix are found in relatively deep locations, and moderate lighting, should be sufficient for these macroalgae to thrive in the home.
Brown algae as a whole are more closely related to unicellular diatoms, than to either red and green macroalgae, including land plants. Yet brown algae are multicellular organisms so similar to their red algae parallels, via convergent evolution, that their fossils may be difficult to distinguish - uncalcified macroalgae are preserved as ambiguous soft tissue impressions or fragments that are subject to differences of palaeontological interpretation. Despite naysayers, there are what seem to be quite definite kelps preserved in the Carboniferous of Illinois.
But as with the seagrasses, the brown algae appear to become ecologically dominant no earlier than the Cenozoic - modern kelp forests are thus a recent biome, as are prairies on land. Despite the reputations of algae as 'primitive' they are evolving as living components of ecosystems, including new Cenozoic habitats. Certainly the Sargasso Sea cannot be more ancient than the formation of the doldrums and the origins of the genus Sargassum; like the continents, the winds and the ocean currents shift over the timescale of millions of years.
The Sargasso Sea features only two species of Sargassum, and both spread by division, not sexual reproduction. It has not a diverse flora, though diverse animals associate with he floating seaweed. The impression given is one of a new ecosystem, one that is slowly forming due to the absence of sexual reproduction. Other macroplanktonic biomes, such as floating crinoid forests, have existed at other times in earth history.