If an aquatic pet is placed into an aquarium volume it can only swim if it is not within the boundaries of a canvas, regardless if collision is disabled.
Create an Aquarium volume and fill it with fish
Create a canvas cube of the same size, give it a water texture and disable collision
Place the volume and canvas on top of each other
The fish to swim around in the block of water
The fish remain permanently frozen in place until the canvas is moved away
The fish inside the canvas are frozen in place, the fish outside of it are free to move
If the volume is moved out of the canvas any fish within the canvas instantly fly to the top of it