I was toying with the idea of creating a highway for fish to travel around my garden. the ideas was transparent plastic tubes which are interconnected at open containers (so fish get some fresh air). some issues were: connecting the tubes to containers, stiff enough tubes, cleaning the tubes, ensuring that tubes do not clog by plant growth, preventing cat damage.
I was working on an aquatic system for fish, formed of a series of aquariums. there would be a waterfall going form one aquarium to the next and a pump returning the water. BUT. what is important is that the waterfall allow the fish to jump back up. i am not sure if there should be a fish ladder or simply a low waterfall. Possibly a fish elevator, which needs to have few moving parts, maybe in the form of a water wheel or a chain of buckets. another thought was an Archimedes screw or even a tube which is filled with water by pump to keep it form emptying. need some way to convince fish to go in… another issue is that any significant height differential encourages the water to empty out rather fast from any permanent opening large enough for fish. closing openings, can pinch fish.
in any case, project stalled due to my dislike of cutting holes in glass aquariums.