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]