Types

• Planktonic algae are made up of thousands of microscopic plants that float on the surface of the water. According to Texas A&M University, these algae are often called "blooms" or "algae bloom," since they color the water green, bluish-green or brown. Bloom algae can consist of a number of species of plants, depending on the light exposure, temperature and nutrients found in the pond.
  
  Filamentous algae are the algae that most people will find on rocks and walls in a pond, or floating on the surface of a pond in long, thin mats called pond scum. Instead of the thousands of plants that make up planktonic algae, filamentous algae consist of single algae cells that form long chains or filaments. When removed from the water, the mats might resemble wool.
  
  

Growth

• Like most plants, algae grow as a result of exposure to sunlight, oxygen and nutrients. You will often find algae in ponds that receive a large amount of direct sunlight and that feature excessive nutrients. These nutrients can come from extra fish food, falling branches or leaves and other sources.
  
  You can often find filamentous algae connected to rocks, walls or floors of ponds, but they do not need a base from which to grow. Algae will grow free-floating in the water, which is how ponds become colored by blooms or covered by pond scum mats.
  
  

Moderation

• Certain types of algae in moderation can actually be good for a pond. Planktonic algae blooms in moderation provide an important basis for the food chain of a pond environment, and many pond owners will promote the growth of algae blooms when they first put fish in their ponds to provide the fish a constant source of food.
  
  Algae blooms can also help to prevent the growth of larger and more dangerous algae below the water. Blooms at the water's surface will provide shade for the pond's bottom, preventing sunlight from reaching areas below. This helps to avoid extensive growth of filamentous algae, which need sunlight to grow.
  
  Submerged filamentous algae can provide breeding and living locations for insects and worms, which can be eaten by fish, thus promoting a food chain. However, fish cannot consume the filamentous algae itself.
  
  

Problems

• Algae in moderation may be beneficial, but in excess they can cause serious problems in your pond. Aside from looking bad, algae can potentially kill your pond.
  
  As stated before, algae need oxygen and nutrients to grow. This means that they are using up these resources, preventing fish and other aquatic life from getting enough. This can be especially important at night, when algae do not receive sunlight. In the day, algae use sunlight to perform photosynthesis and produce oxygen; at night, they have to use the oxygen in the water, which can deprive fish of oxygen and result in their deaths.
  
  

Control

• The most effective way to control algae in your pond is to monitor the nutrients that make their way into your pond. Nutrients come from a variety of sources, including fish and animal waste, food particles and dead vegetation. As these materials decay, they provide nitrogen and phosphorus, which can be used by algae to grow quickly.
  
  When feeding fish in your pond, give them just the amount they need to survive, or even a little less if you want to encourage them to eat bugs in and near the pond. This will ensure that no food is wasted to decay. Immediately remove any dead vegetation, such as falling leaves, from the pond before it has time to decay.
  
  Provide some shade for your pond. The fish will not suffer from a lack of sunlight, but algae will need sunlight to grow effectively.
  
  Algaecides will get rid of infestations of algae quickly, but the algae will always return if you do not solve the problems.