Algae are organisms, not unlike plants, that live in the water. There are a wide variety of algae, some of which are harmful to humans and animals, some of which are completely edible. They are basically colonies of small organisms, some only a single cell.
While we all require algae to live because it creates a large percentage of the Earth’s oxygen, it is generally considered harmful to ponds and water features in gardens. This is because of a vicious and destructive life cycle in enclosed bodies of water.
The algae will grow, consuming carbon dioxide and releasing oxygen. As the algae grow, parts of it die. They fall to the bottom of the water where bacteria and other organisms decompose the organic matter. This decomposition uses the oxygen in the water. De-oxygenated water cannot support most forms of life.
So if there is a sizeable algae bloom, then in time the plants and fish that are in the water are going to die. You only have two options if this begins to happen. You can invest in a pump and attempt to keep the water aerated, or you can kill the algae.
Since algae are really microscopic, just fishing it out with a net will not work. Algae also like to eat the things that help lawns and gardens grow. Algae specifically like phosphorous and nitrogen, two of the three nutrients that are in fertilizer. The more fertilizer that drains into the water, the more algae there will be growing. Before attempting to kill the algae, make sure you have stopped providing it with food. Try not to fertilize around the water, or do not use fertilizer at all.
The key to killing algae is to clean out the water it is growing in, if possible, and then to use enzymes to kill it. Garden and hardware stores sell products that you drop into the water that are full of enzymes who want nothing more than to eat algae.
The larger the body of water, the longer it will take to rid it of algae blooms. The enzymes are very effective. But they are only a first step. Once the water is clean, algae may return. To prevent future blooms, install a water pump that will aerate the water. If the water is kept full of oxygen, then the change that algae will return is very slim.
The use of vacuums and nets and other devices to physically remove algae are not necessary futile, but they do need to be backed up with some more thorough elimination methods.