I had a open top tank many yaers ago. For xmas i got a cute little spotted boxfish. he was happy, always swimming around. then one day he was gone. it happens. didn't think too much about it until one day i was sitting there watching the tv. My cat, a half siamese named furball, jumped up on the arm of the couch and was staring intently into the tank. we thought "how cute" until he jumped up and before i could even yell, had both arms down in the tank fishing......then he met "Mr HAND", and shortly after, learned about "mr Gravity"..........put eggcrate on the next day. no more fishing for furball