I have had my pond for 20 years with no problem, there are still the descendants of the original small koi and two original water lilies. Last year we lost quite a few fish but this year the new generation have come on strong. Early this year we decided to clear out the roots of the water lilies as they had spread to take up nearly one third of the depth of the water. For the first time I now have a floating dark green/brown algae that I skim off every day, multiplying very fast and an oily film on the top. What natural methods can I use to control this? I prefer not to use any chemicals on my property