i live in ohio, and the most common ant i see are the big black kind. they dont bite or sting like a fire ant would, so i dont think venom is an issue, at least in northern ohio. they find their way into the kitchen sometimes, and when i see one, i grab it and the fish love them. i wouldnt throw a dead ant in the tank that might have been affected by pesticides, but a live, big black ant that gets in the kitchen is history. if i had seen a fish harmed by giving them an ant, i would surely warn against it.
edit:
im not meaning to contradict you(fishy), l think fla may have more venomous bugs and i would be more careful if i lived there.