You might be better off googling for an answer, but, in the meantime, ours was much clearer when we had lots of plants floating around - hyacinths, etc.

Had more plants but it attracted FROGS

oops! how deep is your pond? Does it have a sandy or muddy bottom? Maybe the fish are digging around in it?

Oh ya sand

We bought one of those infra-red filters and our little pond water is crystal clear. The pump filter pumps as usual, but the water also goes through an infra-red tube (outside the pond and not submersible) and the water comes out clear. The edges of the pond still gets some algae but the fish eat it. The water is sparkling though.

3ft deep

Aren't frogs good for ponds?

Here's a link to an article that explains why the water turns green and how to treat/prevent it. Saves me a lot of typing :-)
http://uk.answers.yahoo.com/question/index?qid=20070209092237AAICWFY

just remember fish get sunburnt aswell as us and the sun turns the water green, a good protection 4 fsih but plenty of plants give oxygen 2 the water and healthy 4 the fish, small stones(round) layered at bottom of pond will reduce dirty lookn water

My husband uses bleach in our pond to keep the water clear but we do not have fish. We have a bird bath standing in the center and water runs over the birdbath....it is on a timer.

Fish will not survive bleach.

This pond was worse than that .Believe it or not this was done by a seasoned pro landscape company . We Fixed it though, every stone is set in mortar.. If you have no fish, if you have fish they have to be removed and placed in the same water not fresh water. You need to get a wet vac drain the pond ,keep using the wet vac and clean all debris from bottom don't use bleach you disturb the bacteria and algae balance, for those of you who do have fish. Remove the filtration filters and clean them too,. Now fill the pond with water just enough to scrub the walls and wet vac again repeating this process until the water is cleaned ,check to see if the filters need replacing . Now that the water is cleaned fill the pond back up. for those of you who have fish and do this process you need to reintroduce bacteria you can get this at your local pond store or fish stone.

Thanks everyone for all your help :-)