I'd certainly be hesitant to use it too near an aquarium or koi pond; but in all honesty I'm not sure at what concentration any runoff would effect fish. I've never used it near a body of water. If you're talking small scale use (a few plants) you might try using a hand held spray bottle to mist the leaves in a foliar application, or even use a sponge dipped into the solution & wipe it onto the leaves, this would allow you to tightly control any overspray & avoid any significant runoff.
For larger applications, I think your best bet would be to shoot an email to these folks:
GrowOrganic: Fertall Liquid Iron Chelate (Gallon) and consult with your county extension agent to be certain. Best to err on the side of caution I think. Hope some of this info helps. Regards, GF.
EDIT: I just did a little searching & found that there doesn't seem to be much scientific info on the effects of iron chelate on fish, not even from safety data sheets. Sorry I wasn't more helpful. Regards, GF.