FYI - Chemicals released from barley straw inhibit some algae species
https://iahr.tandfonline.com/doi/abs/10.1080/09593331003663294 . Whether "natural" or "synthetic", chemicals can be selective or not. However, humans have not yet made a chemical as toxic to mammals as the naturally occurring botulism toxin. The fallacy that "natural" chemicals are safer than "synthetic" chemicals is very commonly believed by non-scientists, but is false just the same. I personally simply allow my emergent plants and waterlilies to absorb excess nutrients and keep the algae in check. This has worked very well for me.
