Nigel, the resident goldfish, lives in a 30 gallon tank here in the computer room. Nigel is about five years old and close to eight inches long -- quite the formidable fish! However, he's lately required a fortification of his living quarters, as a certain little alpha cat has proven herself to be incredibly persistent in her quest to breach the aquarium's defences.
Observe the bit of Scotch tape here. This is necessary to hold the lid down, because it opens pretty easily with the merest flick of a paw. (Also, Nigel himself has been known to flip the lid up when he goes acrobatically after his food, so it's really important to make sure that can't happen!):
Observe here that the filter/pump cover is taped down at the edges. This is necessary because Cora knows how to remove said cover otherwise. The white plastic thing adjacent to the cover is an after-market add on placed there by yours truly, upon realizing that a barrier was needed to thwart the paw of kitties who seem to be laboring under the fantasy that they are, in fact, raccoons:
The following set of images shows Cora in various stages of her recent attempt to breach the aquarium's fortifications. Fortunately for Nigel (and her as well...all kidding aside, if you have an aquarium and a cat in the same house, it's VERY important for the cat's safety as well as the fish's to make the aquarium into something approximating a fortress), she was ultimately thwarted in her efforts.
But she certainly made a good show of the whole thing, and the photos below are a very good illustration of what it means for a cat to be "constantly calculating angles"!