Aquaponics
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Start with the rule, not the build

Everything else on this page is adjustable. This is not: the bottles must have a documented edible prior-contents history with a wash certificate, or be rebottled with a new virgin bladder. We will not quote a technical-grade tote for an aquaponics system at any price.

The reason is the failure mode. Fish respond to surfactant residues at parts-per-million concentrations, and HDPE releases absorbed residue slowly over months rather than all at once. So a detergent-history tank does not fail on day one — it fails in week six, after you have stocked it, cycled it, planted it and stopped suspecting the tank. That is an expensive and demoralising way to learn about polymer permeability.

Step 1 — the two vessels

• Fish tank: one 275 gallon food-grade tote, uncut, cage intact. Cut an access hatch in the top face only — leave the shoulder and the hoop strength alone.
• Media bed: one 275 gallon food-grade tote, top-cut at about 14 inches from the base on a heated-blade jig, with a reinforcement band because the cut goes below the shoulder.

That gives roughly 250 gallons of working water and about 10 cubic feet of media bed — close to 1:1, which is the conservative ratio to start from. Push toward 1:2 once the system is mature.

Step 2 — the stand and the fall

The grow bed must sit above the fish tank so the return is gravity-fed. A welded stand in reclaimed cage tube at 30 inches puts the media bed outlet comfortably above the tank water line. $520 for a double-width stand, or $180 for a single if you are stacking rather than standing side by side.

Keep the cage on both totes. It is the only reason a forklift can still move either vessel, and once a media bed is full of gravel and water it weighs about 1,100 lb.

Step 3 — plumbing: pump up, gravity down

1. A small submersible pump in the fish tank lifts water to the grow bed. Size it to turn over the tank volume roughly once per hour — about 250 gallons per hour for this build.
2. Distribute into the media bed through a drilled spray bar, not a single inlet. A single inlet channels and leaves most of the bed biologically dead. $165 fabricated.
3. A bell siphon at the far end of the bed floods and drains it automatically — this is what oxygenates the root zone and the bacteria. Set the flood depth about two inches below the media surface.
4. Gravity return from the siphon to the fish tank, through a 2-inch bulkhead. $45 fitted.
5. Add supplementary aeration in the fish tank. The siphon oxygenates the bed; the fish need their own air stone and it is the cheapest insurance in the system.

Step 4 — media

Expanded clay pellets or washed 3/4-inch river gravel. Gravel is a tenth the price and works perfectly well; it is simply heavy, which is why the cage matters. Avoid limestone — it raises pH relentlessly and you will spend the system's life fighting it. Wash whatever you use until the runoff is clear, or you will cloud the tank for a month.

Step 5 — cycle before you stock

Run the system with no fish for four to six weeks while the nitrifying bacteria establish. Dose a small amount of ammonia source, then watch ammonia rise and fall, nitrite rise and fall, and nitrate climb. When ammonia and nitrite both read near zero and nitrate is present, the biofilter is live and you can stock.

Stocking into an uncycled system is the second most common way to kill a first aquaponics build. The first is the tote grade.

Step 6 — stock conservatively

One pound of fish per 10 gallons of tank volume to begin: about 25 lb in a 250-gallon working volume, which is 25 to 30 tilapia at harvest size. Experienced operators run double that with strong aeration and biofiltration. A first system should not.

Step 7 — site it out of the sun

UV embrittles the HDPE and grows algae in the water. A fitted sleeve on the fish tank is $38; shade or a polytunnel is better. Algae is not merely cosmetic — a heavy bloom crashes dissolved oxygen overnight and that is how fish are lost.