Even a well-maintained fountain where the water is changed frequently will develop algae over time. A green film forms on the inside, especially in warmer temperatures. Algae in the fountain can easily be kept under control as long as it is removed regularly. When cleaning, the entire fountain, including the water pump, must be cleaned.

To keep the fountain completely clean so that the algae stay away for a while, it is important to make it as empty as possible. For smaller fountains, you can simply pour out the water. For larger fountains, the water must be drained or vacuumed out. There are a number of special cleaning agents on the market for removing algae. Vinegar also works very well. The cleaning agent is applied to the surface and the algae scrubbed off with a brush. Care must be taken not to scratch the surface. 
An important part of removing algae is to rinse the fountain thoroughly after scrubbing. If the cleaning agent is not completely rinsed away, it can foam up when the water pump is turned on and cause the fountain to overflow. The fountain should be rinsed out with clear water until all greenish residues have completely disappeared. 

Now that the inside of the fountain has been cleaned, the water pump must also be freed from algae deposits. Algae deposits can damage the pump in the long term. The formation of new algae is also accelerated if there are still residues on the pump. The water pump can be scrubbed with a toothbrush so that you can get into cracks and corners. Don't forget to wash the water pipe that is connected to the pump and rinse it with clear water. 

Algae formation can be prevented in purely ornamental fountains by adding chlorine to the water.