Selecting plants as a decoration for a saltwater aquarium can be somewhat tricky since, in most cases, they're not typical "plants" at all, but rather algae. Algae have gotten a bad rap, particularly from freshwater hobbyists who lunge for an herbicide with the mere mention of the word. But algae are nothing more than non-flowering plants that usually grow in water. They are extremely hardy (among the oldest life forms on this planet) and require only good water quality and light, with nutrients being supplied from the waste products of your pets.
For the saltwater hobbyist the distinction between bad and good algae lies between micro-algae, which are only seen once they are growing out of control, and macro-algae, which grow in various shapes and forms. Micro-algae can wreak havoc in your system, but macro-algae can add a nice "natural" touch to your aquarium.
Algae Improves Water Quality
These "plants" are easily thinned or weeded and can help improve the water quality in your tank by taking up nitrates and aerating water. If you have fish or invertebrates that need some vegetable matter in their diet, macro-algae provide a feeding station and hiding places as well.
Because algae don't flower and thus have no seeds, many reproduce either through clipping or a release of spores, which in a tank most often get cleaned out by the filtration system. Algae typically don't need any help once they get going: You'll find you need to prune them eventually, so reproduction isn't commonly an issue unless you've had a die-back. Algae terminology is a little different from plants. What would be identified as leaves in plants are called blades, stems are called stipes and roots are called holdfasts.
Green algae are photosynthetic and need lots of light; many calcareous algae, also a type of green algae, begin as green in color but turn white as they age and calcium carbonate encrusts on their blades and stipes. Red and brown varieties can do fine in low to medium light. Green algae grow fast in brighter light, slower in dimmer lighting, however the red and brown varieties grow slowly regardless of the light levels beyond their normal requirements.
Several popular varieties of macro-algae are available from stores. Some require a little more pruning than others. If you have coral in your tank, you need to make sure that as the algae spreads it doesn't block out living space and light and overgrow your invertebrates. Once algae has taken hold of your corals, it's difficult to get them out, embedding their holdfasts into the coral rock.
Should you decide to add macro-algae to your tank, be aware of "die-back," in which algae turn white and die, in some cases, overnight. This is usually caused by poor water quality, or sometimes just "old age" of the plants. When this happens, the water can turn milky-white as all the toxins are released into it and kill your pets in less than a day. If you detect any sign of such deterioration, be sure to remove the algae immediately or the section of it that is dying.
Varieties of Algae
Live rock commonly has pink encrusting algae and foraminifera on it, both calcareous in nature. Reef keepers love it because it isn't that prolific a grower and adds a little more color to the tank. If you have pink encrusting algae, almost inevitable in most reef tanks, you may have to add a little extra calcium.
Try to stay away from highly prolific growers such as Bryopsis spp. and Seriatopora spp since both will quickly overgrow anything not moving in your tank. For algae-only tanks, these can be a decorative addition.
The only potential problems result in either poor care of these plants or an overgrowth, which can be easily controlled by merely removing them.