Do you get any geese or ducks in your pond?
Grass carp do nothing for the algea. the more carp will make it worse.
Your pond is out of balance. Take a water sample and have tested. you will probably find high phosphate levels in water and nitrogen.
this is my opinion and what i would recommend you try.
I know that grass carp don't eat the algae, I don't buy them for the algae problem... I also have a lot of weeds, last year was quite terrible.
And yes, there are 25-30 geese there pretty much all spring/summer long.
