3 Answers

use filters and pond plants which gets rid of algie

Algae spores are present in all bodies of water just add light (sunshine) and Voila!  The more oxygen the more algae.  If this is a concern buy an algaecide.  Try those blue blocks that bulletman is talking about.

Keep the water flowing as much as possible as well.  Lots of movement is a good thing.

You can use clean water blocks which you break up and put in the water,it keeps the water from going slimy first sign of algae forming. they are obtainable from large hardware stores or live fish store, my better half has a large fairy fountain in the entry of our home, she complained the water was going funny so i bought these blocks which are blue and they work fine, you have to do something soon or your pump will fail.