from the MSU pond management pdf file found here:
http://msucares.com/pubs/publications/p1428.pdf

recommendation is 5-10/acre for weed control. for severe infestations, 10-20/acre are recommended.
i may have to remove them later.

i want to lime my connected pond next year. i will need those carp to control those weeds. but i have to dig the spillway out deeper first. i just dont want to do that until i lime it. even if i dont dig it out, when the rains come, water will overflow and fill the lower pond and some of the carp will probably go with it.