Engineers build robotic fish that swim, dock and recharge themselves with no feeding required

Watching fish glide silently around a tank is one of the most relaxing hobbies a person can pick up, but keeping an actual aquarium alive comes with a long list of chores. Regular tank cleanings, careful feeding schedules and constant water quality checks can quickly turn a peaceful hobby into a demanding routine. Two engineers, Filip and Peter of CPSdrone, decided to solve this problem unusually. Instead of keeping live fish, they built tiny robotic submarines shaped like fish that swim around the tank on their own, interact with each other, and even return to a charging dock when their batteries run low, removing almost every maintenance task an aquarium normally demands.Why low-maintenance robotic fish make sense for aquarium loversAnyone who has owned a fish tank knows the frustration of algae buildup, sudden water chemistry changes and fish that need daily feeding to survive. The CPSdrone team wanted the calming visual experience of an aquarium without any of that upkeep. Their solution involved building micro-sized submarines that look and move like real fish, eliminating the need for feeding, oxygenation or filtration. The only maintenance left is occasional charging, and even that has been fully automated through a docking system, as detailed on the official CPSdrone project page.Engineering challenges behind building tiny robotic fishMaking a convincing robotic fish turned out to be far harder than simply shrinking down a regular remote-controlled submarine. Every single component, from the battery to the motors, had to fit inside a small waterproof shell while staying light enough to float yet heavy enough to remain stable underwater. An Arduino Pro Mini acts as the brain of each fish, receiving commands through a specialised low-frequency wireless link capable of penetrating shallow aquarium water. Four ultra-micro brushed motors handle propulsion, with two motors controlling forward movement and steering while the remaining pair manages depth control, allowing the fish to rise and dive naturally.3D printing and waterproofing the submarine hullRather than using a standard fused deposition modelling printer, which often leaves microscopic gaps between printed layers, the team designed the fish bodies in OnShape and printed them using a Formlabs Form 4 SLA resin printer. Resin printing produced smooth, fully waterproof shells precise enough to create tiny working propellers as well. The hull comes in two pieces sealed together with an O-ring, and UV-curable resin was applied around cable exits to stop water from seeping inside. Even with this careful design, waterproofing needed extra refinement. Pressure testing revealed leaks around several motor cable openings, and two out of five newly built fish failed when water managed to get inside the hull.The inductive charging dock keeps the robot fish fully autonomousTo avoid having to open the waterproof shell every time a battery needed charging, the team designed an underwater docking station that uses inductive charging instead of exposed metal contacts, which would corrode quickly underwater. The concept works similarly to wireless smartphone chargers. Electromagnets gently pull each submarine into the correct alignment before charging coils transfer power, releasing the fish automatically once its battery is full. This single feature is what makes the entire system genuinely low-maintenance, since owners never need to physically handle the fish for routine charging.How an overhead camera and Raspberry Pi control the fish swarmAlthough the robotic fish appear to swim independently, much of their intelligence actually comes from outside the tank. A Raspberry Pi connected to an overhead camera continuously tracks ArUco markers placed on top of each submarine, calculating its exact position and orientation in real time. This data feeds into a PID-based control system that steers each fish toward set waypoints while keeping their movement smooth and lifelike rather than jerky or mechanical. The setup essentially turns the entire aquarium into a tracked environment, where the camera acts as the eyes and the Raspberry Pi acts as the coordinating brain for every fish swimming below.What the future holds for robotic aquarium petsPerfectly synchronised swarm behaviour among multiple fish remains a work in progress, but the current system already allows the robotic fish to cruise around the tank, dock themselves for charging without human help, and even play simple games of autonomous tag with each other. For a project built entirely around replacing living creatures with handcrafted machines, the result captures a surprising amount of the charm that real aquarium fish offer, minus the cleaning, feeding and water testing that usually comes attached to the hobby.