How can I stop algae growing in pond?

How can I stop algae growing in pond? The only thing is....It's in a large avery at home, it has a pump pushing water down a water cascade thing, but it constantly gets full of algae. Naturally because it is in an avery whatever is used MUST be non toxic if ingested by the birds. The birds have a bird bath also with clean drinking water so the pond is not their only water supply, however they do drink from it occasionally. I have tried adding a large amount of salt (Dissolved in boiling water) into the water but that has failed to prevent it from building up again.