Liesl: My former landlady had a heavy piece of glass on top of her aquarium. I am not sure where she got it, but I am pretty sure that you could get a piece cut at a window place or glass shop of some sort. However, it sounds like your aquarium top includes a light source (right?), so this won't help in that regard. If you have the room, put a table lamp (or standing lamp) near the aquarium and turn it on for feedings or whenever you normally have the aquarium light on. That is what my landlady did. She had the heavy glass because her birds kept trying to swim in the fish tank. Not good for fish or birds! I am pretty sure the glass would be heavy enough that a ferret couldn't move it. Hope you find a solution. Inez [Posted in FML issue 0777]