The video below will take you through the steps that we would try to remedy the problem of green water algae in a small pond or water garden. There are a couple of additional things we should mention here too.
You won’t find barley or barley straw noted in the video but that’s a viable alternative to try. It may work well in some ponds and it can work fine with beneficial bacteria.
Also nothing specific was mentioned about pond aeration but there should be some consideration given to this if you’re using a bacteria and not getting great results. It’s not enough to assume that a waterfall or small fountain will always provide great aeration in a small pond. They help, but in very hot weather, they may still be lacking a bit compared to a sub-surface aerator.
Otherwise, the video should provide a fair number of ideas on how to work against green water and get effective results.