You are not very specific so I will assume this plant is in your garden near to a pond but not in it? You need to keep the pond clear of any fallen leaves, flower heads and other debris. A net across in the autumn helps as flowers/leaves can poison the water as they rot. I have one MJ fairly near a pond my fish date back to the pond being built which is coming up to 11 years. So I dont think you need worry.

Rotting leaves etc in a pond can produce gases that are toxic to fish, so as Drc says, keep them out of the pond with netting.

dont have willow or ucaliptus over your pond as the leaves are quite poisonus to fish .it doesnt matter next to rivers but builds up quickly in small water areas.