On Mon, 7 Jul 2014 13:38:00 -0400, Robert Green wrote:

Some animal that's rubbing against the tank looking for water. I assume
it's pretty dry where you live.

It's a pretty "big" animal that can make that type of semicircular swirl,
but, we do see it on two different homes' tanks, so, it's caused by
something.
https://c2.staticflickr.com/4/3883/1...d4a0c232_b.jpg

Both homes would have the same deer, mountain lions, cyotes, fox, bobcats,
and, well, nothing else big other than those (e.g., rabbits & quail
galore!).

There's no rain out here to speak of (this news article said we had 4
inches of rain in all of 2013):
http://www.ibtimes.com/current-calif...ir-way-1548912