Your Grass carp are probably starving since they don't eat, and can't digest, FA or chara. Add 15-20 pounds of Tilapia. Tilapia will focus on the FA and chara first, then begin consuming the organic muck on the bottom. You could even add 30-40 pounds since it is late in the season fo more cleanup yet this year.

FWIW, I'd strongly consider taking out 3 or 4 of the white amur...that is just too many for a quarter acre.