Water plants are not immune from the nasty little aphids, which suck their leaves and stems.
Aphids are tiny brown or black bugs that show up in masses on the leaves of plants.
On water plants, they will usually hunker down deep in the folds of new growth or dot the upper surface. If there are aphids, you're likely to find ants nearby on dry land. Unless you live in fire ant country, the ants aren't a problem, but the aphids are. Obviously you can't spray pesticides into your pond if you keep fish. For immediate removal, dunk the infested plant under the water and hold it there until the aphids let go.