Building a media-bed aquaponics system: parts list and plumbing
A media-bed system has three main components: a fish tank, a grow bed filled with inert media, and a pump that moves water between them. Water flows from the fish tank to the grow bed, percolates through the media where bacteria convert ammonia to nitrate and plant roots absorb nutrients, then drains back to the fish tank. The media serves as both the biological filter and the plant growing medium.
This is the most forgiving aquaponics design for a first build. The media bed handles all three functions (biofiltration, solid waste breakdown, and plant growing) in one unit. No separate biofilter, no settling tank, no raft or NFT channel.
Parts list
Fish tank. 200-400 liters (50-100 US gallons) for a starter system. IBC totes (1000L / 275 gal) are the standard for larger builds; cut the top third off an IBC, flip it over, and it becomes the grow bed sitting above the bottom two-thirds fish tank. For a 200L system, a stock tank, aquarium, or Rubbermaid tote works. Opaque walls (fish stress in transparent tanks). A lid or cover reduces evaporation and keeps debris out.
Grow bed. Equal volume to the fish tank or slightly smaller. A 1:1 ratio (200L fish tank, 200L grow bed) is the baseline. The bed should be 25-30 cm deep; shallower than 20 cm doesn't give enough root and bacteria zone. Materials: a wooden frame lined with pond liner (EPDM, not PVC; PVC liners off-gas plasticizers), a repurposed bathtub, a concrete mixing tub, or a commercial grow bed. The bed must support its own weight when full (a 200L bed filled with expanded clay pebbles and water weighs around 200-250 kg).
Media. Expanded clay pebbles (hydroton, LECA) are the standard. Lightweight, pH-neutral after an initial rinse, and provide excellent surface area for bacteria. 15 mm diameter is the usual size. Alternative: river gravel (3/4 inch, 15-20 mm). Cheaper but heavier and slightly less surface area. Avoid limestone gravel (raises pH and KH uncontrollably) and any media smaller than 10 mm (clogs easily, creates anaerobic zones).
Pump. Submersible water pump rated for the head height (the vertical distance from the pump in the fish tank to the top of the grow bed) plus friction losses. For a typical system with 1 meter of head, a pump rated at 1000-2000 L/hr works. The pump should cycle the entire fish tank volume through the grow bed at least once per hour. A 200L tank needs a pump delivering at least 200 L/hr at the actual head height, not the pump's rated free-flow.
Plumbing. 25 mm (1 inch) PVC for the pump outlet to grow bed. 40 mm (1.5 inch) for the grow bed drain back to the fish tank (the drain needs to be larger than the inlet to prevent the bed from overflowing if the drain restricts). Bulkhead fittings through the grow bed wall or floor. Uniseals are the cheap alternative to bulkheads and work fine in thin-walled containers.
Aeration. An air pump with an air stone in the fish tank. Fish need dissolved oxygen above 5 mg/L; tilapia tolerate 3 mg/L but growth slows. Aeration also helps in the grow bed during the drain phase if using a bell siphon (the draining water pulls air into the root zone).
Timer or bell siphon. Controls the flood-and-drain cycle in the grow bed. See the next section.
Flood and drain: timer vs bell siphon
The grow bed floods with water, holds for a period while roots absorb nutrients, then drains back to the fish tank. This cycle provides alternating wet (nutrient absorption) and dry (oxygen absorption) phases for the roots.
Timer method. The pump runs on a cycle timer: 15 minutes on, 45 minutes off. Water fills the grow bed during the on period; it drains through a standpipe (a vertical pipe in the grow bed that sets the maximum water level) by gravity during the off period. Simple, reliable, no moving parts other than the timer and pump. Downsides: the bed never drains completely (water sits below the standpipe overflow height), which can create anaerobic zones in the lower media. Also, the pump starts and stops 16+ times per day, which wears out cheap pumps faster.
Bell siphon. The pump runs continuously. Water rises in the grow bed until it reaches the top of a bell siphon (a pipe-over-pipe auto-siphon), which triggers a siphon that drains the bed rapidly until the water level drops below the siphon intake, breaking the siphon. The bed then refills and the cycle repeats automatically.
Bell siphons drain the bed completely (down to the bottom of the standpipe), which flushes oxygen through the entire root zone. The pump runs continuously (less wear from start-stop cycling). The downside: bell siphons are finicky to tune. The pipe diameters, bell height, snorkel length, and media guard all have to be right or the siphon either won't start, won't stop, or triggers erratically. Expect to spend an afternoon tuning it with the system running and water everywhere.
For a first build, start with the timer method. It works, it's predictable, and debugging is trivial (is the timer running? is the pump running? those are the only two questions). Switch to a bell siphon later if you want the better drainage and continuous pump operation.
Pipe sizing and flow math
The pump outlet to the grow bed: 25 mm PVC is adequate for flow rates up to 2000 L/hr. For larger systems, upsize to 32 mm.
The grow bed drain: 40 mm minimum. If using a bell siphon, the siphon drain pipe should be 40-50 mm; the bell (outer pipe) should be 75-100 mm. A siphon with a drain smaller than 40 mm may not develop enough suction to trigger reliably.
The standpipe (overflow height control): set so the maximum water level in the grow bed is 2-3 cm below the top of the media. Roots in the top 2-3 cm should stay dry to prevent crown rot in most plants.
Common build mistakes
Undersized drain. If the drain can't keep up with the pump inflow, the grow bed overflows. This is the #1 cause of flooded garages in home aquaponics. The drain pipe should be at least 1.5x the diameter of the inlet pipe. Test the system with the pump at full flow before adding media.
No overflow protection. Add a secondary overflow: a pipe through the grow bed wall at the maximum safe water height, draining to a bucket or back to the fish tank. If the primary drain clogs (root intrusion, media lodged in the pipe), the overflow prevents a flood.
Limestone gravel as media. Limestone is calcium carbonate. It dissolves in the slightly acidic water typical of aquaponics systems and drives KH and pH up uncontrollably. Use inert media: expanded clay, river gravel (test with vinegar first; if it fizzes, it contains carbite and will affect pH), or pumice.
Fish tank too small. Small tanks (under 100L) have no thermal or chemical buffer. A fish tank below 200L swings in temperature, pH, and ammonia faster than the biology can adapt. Bigger is more stable.
Skipping the fishless cycle. The grow bed bacteria colony needs 4-6 weeks to establish before fish are added. Dose the system with ammonia (same fishless cycling process as an aquarium, covered in the cycling guide) and verify ammonia and nitrite both process to 0 within 24 hours before stocking fish.
The system sizing calculator computes the grow bed volume, pump flow, and fish stocking range for a given fish tank size. The running cost calculator estimates electricity (pump, air pump, heater) and feed costs for the planned system.