what kind of algea?

Maybe a month or two. Algaes/cynobacteria is stubbon stuff, especially in newer systems.

 

thanks for the info.

 

You can try and siphon out what you can to remove the algae in the short term. Algae problems are usually caused by aquarium water with excess nutrients like phosphate, nitrates etc. In the long term you may have to examine how you care for your tank. There are some commercial dry foods also contain excess nutrients and other undesirable things like nitrates, phosphates etc so I strain/rinse my fresh/frozen foods before feeding my tank. You may want to consider feeding your fish less often. Also using RO/DI water and a high quality salt with little if any NO3, phosphates etc like Tropic Marin will really help too. Adding a phosphate remover, protein skimmer and increasing flow in your tank plus controling your photo period help should all help too.