Grass Carp & Koi � Both of these fish species eat duckweeds & watermeal. Grass carp (white amur), however, will generally consume other aquatic plants first. The fast reproductive potential of duckweeds allows nuisance levels to build while the grass carp are eating other plants. Also, large grass carp struggle to eat such small plants, which is likely why they prefer more substantial plants to eat. Thus, grass carp are not a recommended control for duckweed or watermeal. If used, only sterile, triploid grass carp are legal for stocking in Ohio.
Koi are a smaller cousin to the grass carp, rarely exceeding 12 inches in length. They are quite willing to eat duckweeds and watermeal and their small size allows koi to more efficiently consume them. They can prevent or reduce a duckweed problem if stocked early in spring prior to duckweed appearing. They cannot eradicate an existing problem as consumption by koi cannot keep pace with duckweed �budding.� No stocking rates are known, but prevention has been attained at 50 koi per acre. Pond owners should buy koi possessing little color value to the koi industry because many koi producers will sell these "low value" koi at cost. One drawback is that koi are vulnerable to predation by herons and large bass and maintaining them in sufficient numbers may be difficult.