Canon SX260 Panasonic ZS15
Either camera will be a good choice.
Both take excellent pictures.
I wouldn't use the difference between 16X and 20X as much advantage.
When you use that much zoom, you have to hold the camera extremely steady to prevent camera shake.
The best thing to do is use a tripod or set the camera on a solid surface for zoom shots over 10X.
Taking pictures of fish from a distance of 5 or 6 feet should be OK.
Remember, a zoom lens loses some light the further you zoom.
So you need a well lit situation. And you cannot use flash for such a shot because of flash glare off the glass enclosure.
The best answer is try it and see. That is how you learn some things in photography.