How do I get rid of algae in my fish tank?

Hi guys. I have a 75 litre heated fish tank, with neons, guppies, etc. I�ve had it about 16 months now and the fish seem happy enough!! The only problem is the tank has developed very ugly green algae, mainly on the white stones on the floor of the tank and on the ornaments inside. I�ve tried various liquids which claim to kill algae but have had no luck so far. I do a small water change once a week (5 litres) using bottled distilled water. Has anyone got any suggestions as the tank looks really ugly!! I�ve thought of buying some darker coloured stones where the algae won�t show up but I�m not sure if this will upset the environment for the fish. When I originally set up the tank I put 1 natural plant in there but have since removed it as I�ve a feeling that�s what caused the algae, but now I can�t get rid of it!! HELP!!!