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

Welcome to the forums V!

I'll take a guess at what your experiencing and then wait for some expert back-up from some of the more seasoned PB members...

First off, you will need to produce some pictures of the "floating dark green/brown algae" so that the stuff can be more positively identified. Close-up and far away pics are good, then some with the stuff on a white background, maybe even some floating in a clear glass with clear water.

Now, my guess is that you have removed the lilies that used to keep sunlight from getting to the floor of the pond. This sunlight is growing filamentous algae (FA) on the bottom that eventually floats to the top a looks unattractive. Manual remove is a common way to help the situation, but will not necessarily cure the problem.

removing the lilies allowed something else (suspect algae) to develop and consume those nutrients. How many lilies did you remove?