Three main types of algae can cause problems in marine tanks: hair algae, slime algae and diatomic algae.
Algae is perhaps one of the most common nuisances faced by marine tank owners---its greenish growth can cover the walls and decorations in the tank, obstructing the aesthetic appeal. Unfortunately, algae is a product of many factors such as brightly lit tanks or high levels of nitrate or ammonia in the water. Algae is very difficult to get rid of once it has spread, and although it is not usually directly harmful to your fish and aquatic plants, it can make your tank and uncared for.
Decreased Water Quality
One of the main problems caused by algae is not the algae itself, but what its growth suggests. Algae feeds off nitrates and ammonia, so when you notice algae in your tank your water is likely supporting the growth based on its contents. Phosphates in the water are one of algae's main food sources, and therefore if your phosphate levels are too high, you are promoting growth. To resolve problems based on water quality, regular testing should be done to test the ammonia, nitrate and phosphate levels---chemicals can be added accordingly to resolve the issue.
Aesthetic Appeal
Many people have marine aquariums because they add an element of visual appeal to a room---fish swimming around in a crystalline tank can add a calming and peaceful feel. However, when the walls of the tank are covered by thick green slime, it often defeats the purpose of owning a decorative tank. Although the algae is often not harmful to the fish, we still want to see what is going on; dark green algae makes the tank look "dirty" as we expect to see clear, blue waters.
Rampant Growth
Algae can actually be helpful to a fish tank as it provides the fish with nutrient rich food. However, you should never let your marine tank become overridden with algae, even if you don't care about the tank's physical appearance. When algae grows rampant, it can actually stifle out delicate organisms in a tank as well as sessile invertebrates.
Removal Process
Another of the problems that stems from algae growth is the process required to remove it. Algae is very unpredictable and there is no cure-all solution to remove it. Often times, tank owners will have to make use of several methods to get the job done, depending on the size and condition of their marine tank. Chemicals to neutralize the water, regular replacement of water and other changes to the habitat can be stressful and potentially harmful to the fish and plants that live there.
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