Aquaponics bell siphon devices drain a media bed automatically, so it floods and empties on a cycle without a pump timer. Water flows into the bed at a steady rate, rises to the top of a standpipe, and starts dripping down it. A capped pipe called the bell sits over the standpipe; as water falls, the bell’s airtight seal creates suction, the siphon kicks in, and the bed drains faster than it fills.
When the water drops to the bell’s bottom slots or air hole, air rushes in, the siphon breaks, and the bed refills. FAO’s manual gives a working design for a 1–3 m² bed, 12 inches (30 cm) deep, fed 200–500 liters per hour (about 53–132 gallons per hour): a 1-inch standpipe, a 3-inch bell, and a 4-inch media guard.
Key takeaway: An aquaponics bell siphon works only when the inflow is strong enough to start the siphon but gentle enough to let it stop. Most problems come down to flow rate, air leaks, or a blocked air inlet.
| Part (FAO design) | Size | Job |
|---|---|---|
| Standpipe | 1 in (2.5 cm) PVC, about 8.7 in (22 cm) tall | Sets max water level; drains to sump |
| Bell | 3 in (7.5 cm) PVC, about 10 in (25 cm) tall, capped | Creates the airtight siphon |
| Bell slots | Two 0.4 × 1.6 in (1 × 4 cm) gaps near the bottom | Let water in under the bell |
| Air hole | About 0.4 in (1 cm), 2 in (5 cm) from the bottom | Lets air in to break the siphon |
| Media guard | About 4.3 in (11 cm) PVC, 12.6 in (32 cm) tall, perforated | Keeps gravel away from the bell |
| Inflow | About 53–132 GPH per bed | Must start and stop the siphon |
How an Aquaponics Bell Siphon Works
FAO describes the cycle simply: water flows into the grow bed at a constant rate. When it reaches the top of the standpipe, it starts dripping down. Without a bell, the water level would just stay there. With the bell in place, falling water pulls air out of the bell, creating suction that starts the siphon. Water then rushes down the standpipe faster than it’s flowing in. When the bed is drained, air enters the bottom of the bell and stops the siphon. The bed refills, and the cycle repeats on its own.
The result is flood-and-drain irrigation that gives roots fresh nutrients and air, which FAO says replenishes oxygen for plants and bacteria. Flood-and-drain beds usually cycle 1–2 times an hour.
Aquaponics Bell Siphon vs Timer
- Aquaponics bell siphon: the pump runs continuously, which keeps water moving through the fish tank and filters. FAO notes siphons can be finicky and need attention.
- Timer: the pump cycles on and off. FAO says a timer avoids aquaponics bell siphon calibration, but reduced circulation means less filtration and aeration, so it’s less suited to heavy stocking and needs extra air for fish.
See flood and drain timing for timer-based systems.
How to Build an Aquaponics Bell Siphon
Sizes depend on bed size and flow rate. These steps follow FAO’s small-scale design and the University of Hawaii’s backyard bell siphon guide.
- Drill the grow bed at the drain point and install a 1-inch bulkhead (barrel) fitting with its rubber washer inside the bed. Tighten both sides.
- Fit the standpipe. Thread an adapter onto the bulkhead and slot in a 1-inch PVC standpipe. Its height sets the high water level; in FAO’s 12-inch bed, it’s about 8.7 inches, leaving a dry top layer of media.
- Add a funnel top. FAO adds a reducer (about 1.5 to 1 inch) on top of the standpipe so more water can start flowing down at once, which helps start the siphon.
- Make the bell. Cut a 3-inch PVC pipe a little taller than the standpipe. Cut two small slots at the bottom edge and drill an air hole near the bottom. Cap the top airtight.
- Make the media guard. Cut a 4-inch PVC pipe taller than the media and cut or drill many slots or holes so water passes but gravel doesn’t.
- Test before adding media. FAO recommends running the system without the siphon first to check leaks, then reassembling and filling with media.
- Add media around the guard carefully, so it stays in place.
Getting Aquaponics Bell Siphon Flow Right
FAO says the inflow must be enough to start the siphon, but not so strong that it prevents the siphon from stopping. For FAO’s design, that’s about 200–500 liters per hour (53–132 GPH) per bed. Use a valve on the inlet to tune it. If the siphon never starts, increase flow slightly. If it never stops, reduce flow.
Troubleshooting an Aquaponics Bell Siphon
The University of Hawaii guide lists the most common problems:
| Problem | Common causes | Fix |
|---|---|---|
| Siphon won’t start; bed stays full | Air leak at the bell cap or air tube; bell not seated; gravel lifting the bell | Seal leaks airtight; reseat the bell and guard; slightly increase inflow |
| Siphon won’t stop; constant trickle | Air inlet blocked by media or roots; inflow too strong; drain pipe geometry | Clear the air inlet; reduce inflow; adjust drain pipe lengths |
| Bed drains partly then stops | Air entering too early | Check air hole height and seals |
| Siphon slows over weeks | Roots or solids in the bell | Clean the bell and guard; trim roots |
The Hawaii guide notes that small changes to drain pipe lengths below the bed often fix a siphon that won’t stop.
Aquaponics Bell Siphon Maintenance
- Check the siphon cycle daily for the first weeks.
- Lift and clean the bell and guard every few months.
- Keep roots out of the media guard.
- FAO notes seeds sown directly in a flood-and-drain bed can wash around, so start seedlings separately.
Bell Siphons and Fish Safety
If your pump pulls from the fish tank and the bed drains to a sump, a failed siphon can lower the fish tank. A sump-based layout, where the pump sits in the sump and the fish tank overflows by gravity, keeps the fish tank level constant. Always fit an emergency overflow in each bed. See aquaponics media beds and pump fail-safes.
FAQs: Aquaponics Bell Siphons
How does a bell siphon work?
Water rises to the standpipe top, falling water creates suction under the bell, the bed drains quickly, and air entering the bell stops the siphon.
What size aquaponics bell siphon do I need?
FAO’s design for a 1–3 m², 12-inch bed uses a 1-inch standpipe, a 3-inch bell, and a 4-inch media guard.
Why won’t my aquaponics bell siphon start?
Usually an air leak or too little inflow.
Why won’t my bell siphon stop?
Usually a blocked air inlet or too much inflow.
Do I need a timer with an aquaponics bell siphon?
No. The pump runs continuously and the siphon controls the cycle.
Sources
- FAO — Small-scale aquaponic food production
- University of Hawaii CTAHR — Construction of Automatic Bell Siphons for Backyard Aquaponic Systems
- Photo credits (Wikimedia Commons, edited with titles): AQUAPONIE by Hopeballa (CC BY-SA 4.0); Hydroton by Lucis (CC BY-SA 3.0); Solanum lycopersicum in aquaponics at the NC Greenhouse Nursery by Gogerr (CC BY-SA 4.0); Scoria Macro Digon3 by “Jonathan Zander (Digon3)” (CC BY-SA 3.0); Hydroton by Lucis (CC BY-SA 3.0)