Two difficult things to do. Remove older GC and take care of an invasive aquatic plant.

Grass Carp being as skiddish as a bonefish make it difficult to remove them once they have grown up. I have heard some people use the bow fishing method with the best results. This allows you to stand back far enough not to scare the fish yet catch them.

The plant without an actual picture sounds like Bladderwort. If you are okay with using an aquatic herbicide, I can make some recomendations on what to use after a firm identification is made.