Ixnay-
That's the spot I pointed out in the bug reports (which have now disappeared) some time ago. I called it "opaque gravel" because when a fish swam through there ( it has to be behind the rocks) you could see a cloudy image (of the gravel?) superimposed on the fish.
I only saw this a couple of times, because you have to wait so long for a fish to swim through there. It may have been fixed (or I may have imagined it
); but keep an eye on that spot and see if you can see it.
BTW, I now have v1.0 !!!!! What an amazing screensaver.