Hi sirbatta,
Algae only come if too much nutrients are present in the water.
I don't know about temperatures where you live but keeping a tank outside there are some considerations.
First no rain should enter the tank, second you must be able to keep tehe temperatures in a what is considered optimal range. (between 23-27°C)
Also have to have outdoor electrical circuit.
I also have a temperate North sea aquarium of 1200 l outside, in my case living in Belgium I have to take measures in summer against the heat and in winter so the water doesn' t freeze.