A few weeks ago I was over a friend's place and in each of his tanks there was a cotton lead line tied to a fence post and draped into the tank. When I asked why he told me about the same problems he was having with animals looking for a drink and not being able to get out. The rope was there for their use in climbing out. He claimed that since he put the rope in there, there have been no more animals to scoop out.
No more dead ones for me either since I put a rope in mine. Just thought I'd pass that along.