Killing weed

Hi, fished my favourite lake the weekend and had a bad one, very few fish out. It could of been due to it being very hot or just one of them days, or, it could of been that the owners are trying to kill off some of the weed that's a little problematic in the lake. Before I finished my session I see the bailiff out in a boat with some sort of dispenser shaking in some grey coloured power which I believe was weed killer type stuff. Now I'm wondering that if they added some last week as a first dose as if to say would it, or can it upset the fish in some way?

I don't know much about the chemicals that can be added to a lake to remove weed so anyone care to give me a little insight?

Thanks for your time.

i think you would need to know what the chemical was and what effect it has on water quality and oxygen levels.most of the chemicals used are plant specific and made for stocked ponds so i cant see it effecting the fish not in the long term at least.