Aquatic plants are used to give the aquarium a natural appearance, oxygenate the water, and provide habitat for fish, especially fry (babies) and for invertebrates. Some aquarium fish and invertebrates also eat live plants. Hobby aquarists use aquatic plants for aquascaping.
Marine algae are also included in this list for convenience, despite the fact that many species are technically classified as protists, not plants.
Brown macroalgae are sometimes seasonally available in the aquarium trade and under suitable conditions, will grow quite prolifically. They possess the pigment Fucoxanthin which gives them their coloration ranging from yellow to dark brown. A few are desirable yet many are pests with some species being rather difficult to remove, often making their way into aquariums on live rock.
Green macroalgae are extremely diverse and abundant, coming in a wide variety of shapes. They possess the pigment Chlorophyll which gives them their coloration ranging from bright green to yellow or dark jade. Reproduction via fragmentation or the releasing of spores are utilized by this macroalgae group. The second option, which is used by the genus, Caulerpa, can be quite problematic in an aquarium. Some species of green macroalgae have greater ease surviving in high nutrient environments or inadequate water parameters than other kinds of macroalgae and are generally the most suitable for nutrient removal due to their rapid growth rates.
Red macroalgae are highly sought after in the aquarium trade given their intense coloration and striking appearance. They possess the pigment Phycoerythrin which gives them their vivid coloration ranging from red to orange to blue. For some species, identification is difficult and slight variations due to location and depth may exist.
Most mangroves that end up in the aquarium trade are either collected as seeds or grown on land. Their leaves should be exposed to the air, well beyond the surface of the water, and be sprayed with freshwater regularly to prevent salt buildup. They also require intense lighting and a layer of substrate, not too shallow as to let the tree topple over and not too fine which would make parts of the sandbed to go anaerobic.
While algaculture is more popular in marine aquaria than in a freshwater setting, there are several species of algae that are unwanted or problematic. These species can be very difficult to remove once established. They are disseminated in the aquarium trade through improperly collected or processed material, such as live rock.
While there many species that may be referred to as "hair algae", species in the genera Bryopsis and Derbesia are among the most common. Bryopsis plumosa is especially troublesome. Boodlea species are occasional in marine aquaria, and may be called "crunchy hair grass". Red hair algae (Polysiphonia) may also be a nuisance.
"Turf algae" is a broad classifications of algae, and refers to their branched leafy or lawn-like growth habit. Green turf algae include the genera Cladophora (and potentially Cladophoropsis) and Ulva, as well as the species Chlorodesmis fastigiata.
Brown turf algae often arrive on contaminated live rock and include the genus Dictyota as well as the species Lobophora variegata and Padina australis.
Common genera of red turf algae include Galaxaura, Gelidium, and the more common species include Nitophyllum punctatum, Wrangelia argus, and Centroceras clavulatum. Although sometimes desirable, Hypnea musciformis may be considered a pest.
Bubble algae can grow large and spread rapidly. The red bubble algae Botryocladia skottsbergii is especially problematic and difficult to control. Green bubble algae of the genera Valonia, Ventricaria and the species Dictyosphaeria cavernosa may become nuisances.
The green microalgae Ostreobium is parasitic in stony coral skeletons and compromises the structure of the animal. Brown diatom algae and dinoflagellates are also ubiquitous in marine aquaria. Blue-green algae (cyanobacteria) may also be present, with Phormidium corallyticum causing black band disease in coral.