What are these plants ? Where do they come from ? Why are they growing in my tank, and how do i control them or get rid of them ? 

Algae, seaweed, or sometimes incorrectly refered to as plants, are organisms that produce their own food by a process called photosynthesis. Terrestrial plants also photosynthesize but the difference in algae is that they lack any vascular tissue, and that they reproduce using spores. There are many forms of algae. These range from the kelp to caulaerpa to those brown dots growing on the panels of glass in your aquarium.

These algaes are introduced into the aquarium by many means. Most often they come in with the fish. Many fish feed upon algae and as they are introduced into the aquarium they deposit what they have previously fed upon via their waste products. In the reef tank live rock, live sand, and living corals bring along many hitch-hiking algaes. It is even possible that the water used during a water change can introduce algae spores into the tank.

Algae, like any other plant, requires three magical ingredients.... light, water, and nutrients. All aquariums have these ingredients and given the right opportunity algae will grow. All aquariums have water. Fish systems generally have basic lighting and high levels of nutrients. Reef systems usually have very high lighting and lower nutrient levels.

Controlling algae can be difficult. The following information should give you some insight into what algae is, and what you can do to minimize its growth.
 

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