Algae is a very large and diverse group of organisms, ranging from tiny unicellular algae to huge organisms such as the Giant Bladder Kelp (Macrocystis pyrifera) that can reach a length of 65 meters. Algae is simply the Latin word for seaweed, but in English the term seaweed is usually only used for the most complex forms of marine algae, e.g. kelp, sea lettuce and brown brains. Most, but not all, algae have a photosynthetic machinery derived from cyanobacteria.
Since the ocean is filled with a myriad of different algae, there are also a lot of different algae that can live in marine aquariums. Within the hobby, aquarists tend to distinguish between macro algae and nuisance algae. Of course, it is up to each aquarist to draw the line between desirable and undesirable inhabitants of the aquarium.
Macro algae can be very beautiful and are often used as decorative algae in aquariums. Examples of popular types of decorative green algae are Maiden’s Hair from the genus Chlorodesmis and Ulva Lettuce Algae from the genus Ulva. Members of the genus Chaetomorpha (commonly known as Spaghetti algae, Green Hair algae or simply Chaeto) form dense green mats in the aquarium and are known to be sturdy and easy to keep. They are even added to refugia to serve as filters.
If you prefer red algae, you can for instance try some red kelp from the genera Haliptilon and/or Botryocladia.
Encrusting coralline algae can be a nice addition to reef tanks since these algae form carbonate shells, making them somewhat similar to coral. Getting encrusting algae to thrive in the tank can be tricky, but if you manage to succeed you will be rewarded with good covering and a nice splash of color in the aquarium. Encrusting corraline algae come in a wide range of hues, including green, red, purple, pink and white.
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Nuisance algae can be a major problem in marine aquariums, especially ones with plenty of natural light or powerful artificial lightning. Before you embark on any eradication quest, it is a good idea to try to diagnose which type of algae you are dealing with. You may not be able to find out the exact species, but you can at least get an idea of what type of algae it is and what makes it thrive. If you can’t adjust the light levels, you may have to starve the algae by carrying out more frequent water changes, feeding less food (or feeding a different type of food) and upping the filtration. Introducing scavengers in the aquarium can also work, as well as introducing desirable algae that will compete with the nuisance algae for nutrients.
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Marine algae feed on nutrients in the water and can therefore help you keep the water quality up in your saltwater tank. Also, nearly all algae produce oxygen as a by-product from photosynthesis while also removing carbon dioxide from the water. There are even algae that will bind heavy metals and various toxins. Of course, if the alga dies the things it absorbed while living can be released into the water again, so removing dead algae (when possible) is recommended. Read more about how to use algae for your filtration.