Pond water quality

Question
Hi
I live in Spain and have constructed a small pond, about 1500 gallons. I have installed a biological filter that was supplied by the local pet store(it is only a small store and does not carry a large selection) this seemed to work for a while but now the water has turned a blue-green colour and there is a lot of bubbles on the top of the water. The goldfish that I have look and seem to be well and have bred well over the last couple of months. I have a couple of water lillies and some oxygenating plants. There does not seem to be any algea growing in the water. I hope this gives you enough imformation and hope you can suggest something.
Thanks
Gordon

Answer
Hi Gordon,
Thanx for your question.  The fact that your water is turning color means you have algae in the water.  I would recommend using water hyacinth, water lettuce, azolla and other floating plants that have extensive root systems that very effectively clean the water.  Additionally, use the brick-sized barley bales which also help keep down algae in the pond.  I hope this helps.
Tom