Standing water can be a breeding ground for a bunch of critters. I have no idea what to do other than flush it off once in a while but one idea that comes to mind is adding something to the water to either kill or discourage them. I have NO IDEA if cover damage can occur but what about a few drops of oil that will form a thin molecular film or maybe borax or baking soda. Maybe RV antifreeze or the older green stuff ethelyne glycol. Just some crazy ideas and I'd like to hear comments too.
Al