Thanks for all your suggesions...
Tsoomi Jumped from the kitchen table on top of the fish tank and fell inside after breaking the lead... that's what happened. It's a huge fish tank (1.5 meter wide, 400 liters) Anyway, we found a solution ... We bought a different one that has a much stronger lead, and will not fall and break if and when one of my cats jumps on it. Now we have to find a buyer for the other one...
Hope this will be the end of our problems...