Start with googleing "free animated fish images" or "free animated fish gifs".  That will get you a good selection of free animation, such as the ones I use on my website.

The html/javascript codes can be found on http://www.htmlfreecodes.com/ .  They can be added to posts when you switch to the html tab at the bottom of your post.

All you have to do is copy and paste the animation from a website to your reply space.  Don't try to resize the animation, as that usually causes the animation to stop working.  Also, make sure that the animation is free to use.  Don't want to be sued!