It's my understanding that it's the surfactant used with Roundup that is harmful to amphibians and other aquatic organisms, that's why it's not labeled for aquatic use. It probably won't kill your fish unless it kills a strong algae bloom and causes a severe dissolved oxygen sag, but it could do some damage to your aquatic food chain. One of the things it might do to your fish is cause liver damage.
Roundup will most likely not/not labeled to kill algae to my understanding. Only weeds etc out of the water like cattails. They recommend using a surfactant with rodeo but never say anything about using some special 'aquatic' surfactant so I'm confused about this whole situation and said to heck with it, I'm going to take the stance that the only difference is rodeo does not have a surfactant and use regular roundup. Not saying I'm right, just my .02 after doing a lot of reading.
