Is it time to add algae to your koi diets?  Here is a research article on this subject.
 
"Fish has long been a source of “rich food for poor people” and has played an important role in increasing food security and nutrition in developing countries. Because various chemicals in algae can have confusing effects, the results of experimental research can be difficult to understand. Algae has been associated with strengthening immune systems, lipid metabolism, antiviral and antibacterial action, improved gut function, stress resistance besides providing a source of protein, amino acids, fatty acids, vitamins and minerals, and other biologically active phytochemicals in cattle and aquaculture feeds, even when used in modest amounts. The addition of algae to the fish diet modified the growth performance of the fish, causing it to improve." 
 
https://www.intechopen.com/chapters/83414