It is common to use pond netting to repel fish-eating predators (such as herons or cranes) and to keep ponds free of leaves and debris. This strong 3/4" mesh woven netting is available in sizes from 6' x 9' to 33' x 20'. It will not rip easily and is an extremely good value.
Woven from UV safe mono filament
Comes with securing stakes
Rot-proof and extremely strong
It is also possible to use a large beach ball or other floating object to create a tenting affect to further aid leaves blowing off of the cover instead of collecting on the cover.