Twin-Tank U-Pipe Aquarium Bridge: an Ecological Balance Experiment

Inspired by curiosity about ecological engineering from biology class, I built this unique twin-tank U-pipe aquarium system. All materials can be easily sourced from the mass market.

1. Basic Construction Tools:
2. Cutting tool (Hacksaw / Plastic pipe cutter) - For cutting PVC pipe.
3. Drilling tool (Power drill / Hand drill) & Appropriately sized drill bit - For drilling a hole in the U-pipe.
4. Measuring tool (Tape measure / Ruler) & Marking pen.
5. (Optional) Sandpaper - To smooth pipe edges.
6. Cleaning tools (Cloth / Brush).
7. Core Structural Components:
8. Transparent square/rectangular tanks x 2 (I used approx. 20x15x15 cm)
9. Transparent rigid PVC pipe (I used approx. 75mm diameter)
10. 90-degree PVC elbows x 2
11. Barbed hose connector x 1
12. Pipe clamps with saddles/bases & fasteners
13. Lego bricks (For base, support, feeder)
14. System Operation & Ecological Components:
15. Ecological substrate (Bioclastic substrate/coral fragments - I collected from Bioclastic Beach, Pumice)
16. Hand-operated bulb pump (siphon starter)
17. Thin flexible tubing
18. Small inline plastic valve x 1
19. "Seed" water
20. Commercial beneficial bacteria blend (including denitrifying bacteria)
21. Ornamental fish
22. Auxiliary equipment (Air pump + air stone)
23. Water quality test reagents (mainly pH, ammonia nitrogen, dissolved oxygen)

One day, while casually flipping through an old copy of "The Boy Scientist" (part of the "Popular Mechanics" magazine project collection series), the inspiration struck. The picture shows the original magazine.

This wasn't just about recreating an interesting design, but more about exploring the possibility of achieving partial ecological cycling in a small, homemade system. Initially, I naively thought the U-pipe structure could slow down water quality changes – of course, the laws of diffusion don't quite work that way in reality.

This article aims to introduce the structural design, operating principles, and some ecological considerations behind this aquarium. It's more of a personal project showcase and experience sharing than a strict step-by-step guide for replication. I hope this introduction inspires you to combine your own ideas and materials to create your own version.

1. The foundation of the system consists of two transparent square (or rectangular) glass or acrylic tanks.
2. Dimensions are flexible; I used containers approximately 20x15x15 cm.

The key connection between the two tanks is a U-shaped pipe made of transparent rigid PVC, creating a visual "aquarium bridge." I used pipe with a diameter of approximately 75mm.

This U-pipe is assembled from three straight sections and two 90-degree elbows. I found the friction-fit connections to be tight enough, so I did not use extra glue for sealing. This ensures basic airtightness while allowing for easier disassembly for cleaning or adjustments later.

Hose Connection Port: Off-center at the U-tube's top, I drilled a small hole to install a barbed hose connector. This connector serves as the crucial interface for attaching external tubing to start the siphon.

The U-pipe is supported by a pipe clamp system.

The base is constructed from Lego bricks. The saddle bases of the pipe clamps are integrated into the Lego base. Above this, a small platform is supported by Lego structures, currently holding only the feeder. More components could be added later.

Designing this seemingly simple device took quite a bit of thought. I initially attempted a conveyor belt design but found it ineffective for fine fish food pellets. I eventually adopted a simpler funnel-slide structure. Precisely controlling the dispensing of small pellets is an interesting design challenge in itself.

The entire system is placed on a sheltered balcony. This location provides suitable natural diffused light while preventing direct rain from interfering with the tank environment.

1. To simulate a natural aquatic environment and provide attachment sites for microorganisms, the bottom of both tanks is covered with a layer (approx. 1-3 cm thick) of cleaned bioclastic substrate (including coral fragments) collected from Bioclastic Beach. This material has a porous structure, greatly increasing the surface area available for beneficial microbes like nitrifying bacteria to colonize.
2. Additionally, some Pumice stones are placed in the tanks.

1. After filling both tanks with a moderate amount of water, ensure the openings of the U-pipe are submerged below the water surface.
2. Attach a length of thin flexible tubing to the barbed hose connector on top of the U-pipe. Connect the other end of the tubing to a hand-operated bulb pump (siphon starter).
3. Starting the Siphon: Use the hand pump to draw the air out from inside the U-pipe. Once water is pulled over the highest point of the U-pipe and begins to flow, the siphon is successfully established. Atmospheric pressure will then automatically keep the water levels in both tanks balanced and connected via the U-pipe.
4. Maintenance Access: At the end of the thin tubing connected to the barbed connector, I installed a small inline plastic valve. After starting the siphon and removing the air, this valve can be closed to prevent air from slowly entering or water from evaporating. This valve and tubing setup also makes it convenient for water change or purging any small amounts of air that might accumulate at the top of the U-pipe.

While aiming for natural circulation, some auxiliary equipment is useful during the initial setup phase, when adjusting stocking levels, or for emergencies. The hand-operated bulb pump used for starting the siphon can also be used to draw water from the top for water changes. An air pump with an air stone can be used for direct oxygenation.

This is the cornerstone of a stable system. Whether through natural colonization or adding bacteria cultures, sufficient time (usually several weeks) must be allowed for nitrifying and denitrifying bacteria (if added) to multiply on the substrate and porous media. They are responsible for converting toxic ammonia/ammonium (from fish waste) into less toxic nitrite, and finally into much less toxic nitrate (or removed via denitrification). Consistent water quality monitoring is needed to confirm completion (ammonia nitrogen readings persistently near zero).

Placing the tank on the sheltered balcony provides moderate natural diffused light. This encourages the growth of beneficial algae on the tank walls and substrate. These algae act not only as natural "water purifiers" (absorbing nitrates and other nutrients) but can theoretically serve as a partial food source for the fish. Additionally, algae perform photosynthesis during the day. My observations confirm this: once the system stabilized, even without auxiliary aeration, the dissolved oxygen level naturally maintained a healthy 7-8mg/L, up from an initial ~5mg/L.

1. The Ultimate Ideal: To create a micro-ecosystem that can sustain life activities long-term without artificial water changes or feeding, relying solely on internal material cycles and energy flow. This is undoubtedly challenging.
2. Current Practical State: After some time running and adjusting, the system shows good stability: ammonia nitrogen is near zero, and the pH is stable around neutral. However, I remain cautious about whether the naturally occurring algae alone are sufficient to fully meet the nutritional needs of the goldfish. Therefore, I currently still perform intermittent feeding and continuously monitor the fish's condition and water quality. Water change frequency has been significantly reduced, performed only when necessary and typically involves small volume changes.

Maintenance is also an ongoing process of observation and learning. One interesting observation: on bright, sunny days, the fish seem particularly fond of congregating in the U-pipe aquarium bridge.

Initial regular microscopic observation of water samples also visually confirmed the abundance of bacterial colonies and single-celled algae or fauna.

Establishing an ecosystem is full of variables. Treat this process as an experiment, allowing for failures and adjustments. If water quality issues arise, have a backup plan (like a temporary container and clean water source) to move the fish temporarily while adjusting the system.

1. The whole structure is a dead ringer for the Pi symbol (π), built using circular pipes (hello, π!).
2. The fish themselves (Pisces) carry the "pi" sound, and even their respiration involves π bonds.
3. Plus, you could argue the self-contained system is like a complex pie – a little world held within!
4. After all, All Things Pi.