OK, guess I've been a bad boy, but I've used RoundUp/similar on Lilly Pads with excellent results for 10 years now and have never had a die off of any fish.
Of all the spray out there RoundUp is the most harmless. I think I would try to stay away from the quick burn stuff that has allot of salt in it.
I think you would have to dose the pond enough to change the PH before it would do any damage and that would be allot of round up.
Cheers Don.