Keeping fish tank clean?

I need some advice on my 200lt fish tank. I clean it thoroughly once a fortnight. I drain 3 quarters of the water, scrub all the plants and ornaments (all fake ones) vacuum the rocks re fill. I use water conditioner and some other stuff but after 1 week everything is completely green again! The water is still clear just everything else is covered in algae. I'v tried algae killers and always check the PH levels are correct but it still happens. I'm about to give up! Please help!