I fought green water for years. The last few years I have solved the problem and I believe some of the reason for my good fortune is the addition of an UV light. It takes a while for it to work and needs to be run continually. It not only gets rid of suspended algae but helps with fish parasites. My fish would always get sick in the spring until I kept the UV going all winter.
I also stopped flushing and trying to clean my biological filter. I only disturb the filter once or twice a year buy flushing it with pond water. I scoop floating debris and try to keep my skimmers emptied too.
I also use a dry bacteria inoculant to keep the beneficial bacteria population high with every partial water change. I'll try and post a picture of the container.
My pond has a layer of what I call a 'bio-film' coating the inside. I never scrub it off. The fish nibble on it but I think it aids in keeping the beneficial biological activity that keep the pond balanced as much as possible.
