I did (finally) find the screensaver I was looking for at
http://www.serenescreen.com/index.asp
I think the screensaver is a lovely work of art, and indeed I will be going ahead to make the purchase (it costs 21.95) right after typing out this post. I think the price is worth it!!
I had downloaded another aquarium screensaver a few days ago that was a freeware, but it did not live up to my expectations.
Thank you all for your help and involvement in this discussion.