As I discovered the other day, there is no such thing as an escape proof aquarium. All of our research aquaria are sealed. They are plumbed with balkhead fittings, the lids are glass on glass with no gaps, outflow is protected with sponge, etc. However, the other day I was photographing a large (.5 kg) Octopus rubescens that we have reared from a new recruit. I lifted the lid to add a shrimp and before I could do anything, the animal climbed (it looked more like a leap) out of the tank and landed splat - 5 feet below on the epoxy floor. The animal survived, but it has now learned about the lid lifting opportunity and I have to go to great deceptive links to tease the animal to come to a bottom corner of the aquarium when I want to add a piece of food.
Roy