Algae arer very simple plants and need the same things are more complex ones. Nutrients and light. Since the light is kinda necessary lets look at the nutrients. Each algae has a slightly different preference on the exact type of nutrients they prefer to use as fertilizer, but they all need nitrogen and phosphorous. Algae blooms occur either when lights are changed or a new mixture of nutrients are added to a tank. These new conditions allow a new type of algae to thrive. I your case a really disgusting green variety. From here you have two options, you can either figure out what is causing the algae to bloom(what nutrients are in your tank) and get rid of them or you can wait jout the algae. In both cases the nutrients the current algae are using will be used up or eliminated and then you can enjoy your tank again. You can try adding a tang or algae blenny to help remove the algae, but this will not stop the problem at its roots.
Some basic ways nutrients come into the water are foods and top off water. Flaked food contain a ton of phosphates and are common culprits of algae blooms. All food however add nutrients to the water, so you have to pay attention to just what type of foods you use and how often. Also the water you use for evaporation can have nutrients in it. Do you use R/O or D/I water? If not you might want to try these types of water and see if they help at all. Tap water in some areas contains many fertilizers, nitrates, phosphates, and silicates that will all help algae grow in your tank.
I hope this helps. Only a lucky few are ever able to win the war with algae, but the idea is to use every advantage you have to help control it
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