Robotic Submarine Fish Are the Ultimate Low-Maintenance Aquarium Pets

Watching fish and other aquatic creatures silently glide around an aquarium provides endless hours of entertainment and relaxation. But keeping an aquarium isn’t all fun and games. There is a lot of maintenance that comes along with it as well. Regular cleanings are a huge headache, and the creatures inside the tank won’t be too happy if they aren’t fed regularly. So, if you don’t want all your fish going belly up, lots of new chores come along with an aquarium.

That is, unless your aquarium is like the one built by Filip and Peter of CPSdrone. They wanted the benefits without the maintenance, so they replaced fish with microsubmarines that look like fish. It may not provide quite the same experience, but it is entertaining in its own way — and aside from the occasional charging (which is automated!), there is no maintenance.

Making these robotic fish look convincing was much more difficult than just shrinking down a remote-controlled submarine. Every component had to fit inside a tiny waterproof hull while remaining light enough to float, yet heavy enough to stay stable underwater. An Arduino Pro Mini serves as the brain, reading commands sent over a specialized low-frequency wireless link capable of penetrating the shallow water of an aquarium. Four ultra-micro brushed motors provide propulsion, with two controlling forward movement and steering while the other pair handles depth.

Rather than relying on a typical fused deposition modeling 3D printer to produce the hull, which can leave microscopic gaps between layers, the team designed the fish in OnShape and produced the parts on a Formlabs Form 4 SLA resin printer. Resin printing created smooth, waterproof shells that were detailed enough to produce tiny propellers as well. The two-piece hull seals with an O-ring, and UV-curable resin was used around cable exits to keep water from sneaking inside.

Even with that design, waterproofing took more work to perfect. Pressure testing revealed that several submarines still leaked around the motor cable openings. Out of five newly manufactured fish, two failed because water found its way into the hull. Tiny amounts of epoxy also seeped into some of the brushed motors, creating speed differences that later complicated coordinated swimming.

To avoid opening the waterproof hull every time a battery needed charging, the pair also designed an underwater docking station. Instead of exposed electrical contacts that would quickly corrode, the dock uses inductive charging coils similar in concept to wireless smartphone chargers. Electromagnets pull each submarine into the correct position, aligning the charging coils before releasing the vehicle once its battery has been replenished.

The fish may look autonomous, but much of the intelligence actually lives outside the aquarium. A Raspberry Pi connected to an overhead camera continuously tracks ArUco markers mounted on top of each submarine, calculating its position and orientation. That information feeds a PID-based control system that guides the fish toward waypoints while keeping their motion smooth and natural.

Although perfectly synchronized swarm behaviors remain a work in progress, the robotic fish can already cruise around the tank, dock themselves for charging, and play games of autonomous tag. For a project that replaces living fish with handcrafted robots, that's an impressive catch.