Hello Oz and others,
All the way from Holland this answer: see for advise www.adahofman.nl . Most of the text is regrettably in Dutch, but maybe it can be translated. She is well known in Holland as a pond-specialist and is specialised on clear water-ponds without using any chemical, straw or filters. When your pond is already green her advise is "start again", remove all the water, plants and everything. Then start again with fresh tapwater (rainwater is far to acid), depending the hardness of your tapwater (in Holland the DH varies from > 12 tot < 4)) she ads "maerl" when DH < 12.
Then - within 24 hours - you have to plant oxigen-plants: she advises especially Potamogeton Lucens, at least 4 bushes or 1 bucket/ m3 along with some other oxigenplants. Plus add a bucketfull of water from a healthy - clear - pond (we got it from Ada Hofman together with the oxigen plants we bought), she calls this a 'enting' (= grafting) which contains the necessary 'healthy' micro-organisms (which tapwater has not)
Then wait and see ... Do not use filters-UV-light etc.
In her botanic aquatic garden in the Netherlands, near the German border, with some 50 ponds (from very large to very small) she demonstrates that it worked. In my own garden (near Utrecht) we followed her advises and we have a clear water pond since 2009. She has written a couple of books about this, but I think it has not been translated in English.
Joke (= a dutch name, it comes from Johanna)