Macroalgae are the larger 'plants' of reef tanks. These are all forms of algae and are not true plants. The term 'macroalgae' describes algae that are large enough to see by eye. Most macro algae can be categorised into four life forms:

1. Fleshy algae.
2. Calcareous algae (which have a calcium carbonate skeleton).
3. Turf algae (which form a mat on the substratum).
4. Crustose coralline algae (which form flat plates on the substratum and may cement coral fragments).

Macroalgal species are divided among three large groups (green, brown and red algae), which are named according to the color of their dominant photosynthetic and accessory pigments.

Green algae (Chlorophyta) contain chlorophyll and are well represented in the tropics. Some of these algae favour stressful environments where nutrients are high and herbivory low. Others are calcified and contribute heavily to the sandy sediments of reef areas.

Brown algae (Phaeophyta) predominantly contain the brown pigment, fucoxanthin, but colonies may range in colour from beige to almost black. In temperate seas, brown algae can form vast kelp forests but their abundance and diversity in tropical seas is reduced. Some of the most common tropical genera include Sargassum and Turbinaria which are often associated with reef flats, and Lobophora which is fairly ubiquitous.

Red algae (Rhodophyta) are the largest and most diverse group but arguably the least well understood. They contain large quantities of the pigment phycoerythrin, which can often resemble the pigmentation of brown algae (fucoxanthin). Red algae are extremely important reef-building organisms, which may form reef crests and large calcareous plates.