Grass Carp ( White Amur) will not breed in still water, they need a briskly flowing turbulent water to successfully reproduce.
Common Carp (Cyprinus Carpio) will redily reproduce in still or flowing water.
I have a 3/4 acre farm pond with 5 carp and they have been in my pond for over 4 years. and all 5 carp are about 30 inches long and I have yet to have a problim with the carp killing or even hurting other fish in my pond.only time my pond isn't clean is when I cut the grass around the pond and the mower throws the gass into the pond. but you can set their with a stop watch and time the Carp. I mean with in seconds they are doing their job. Fun thing about having Carp in your pond is that they are a blast to ctach on a FLY ROD! last time I got to hook into a Carp with my fly rod it took me about 30 to 40 min. to get it into the bank. it was a blast and a rush. but it tired me out way before the carp got tired... Fishing for carp with a regular spin cast type setup it's nowhere as fun as using a fly rod.
TRY IT SOMETIME!!!!