Are they both inside? I would imagine your cat definatley thinks about it pretty dern hard. I have not seen one yet to kill a fish in a tank. Is the fish in a bowl? Cat could dip its paw like we have all seen on tv and stuff to reach the fish. That could be fatal to the fish.
Well I think they do thats why anyone I know with an outdoor pond has a net covering it to keep the cats and birds away from eating the fish.
Some do and some don't. Cats tend to be pretty finiky eaters