Oh, this is so sad and such a tragedy. I'm sorry for you. I have often found mice and moles dead in my skimmer after winter is over. They must just fall in there and drown and I don't know any way to prevent it. I have netting clear into there but they fall in some how. I have often seen squirrels, groundhogs and possom's come and get a drink at my pond and if they fell in I'd probably never know it happened until I found the tragic results. I have no way to put any type of barrier up around the water. They come because they need to drink. I never thought of anything larger than a mouse falling in. I'm sorry this happened to you.