Spring Water, Surfactants and a Tank That Failed in Week Six
Reading this because you are specifying, buying or selling containers? Send the details and a grader answers in writing, usually the same business day.
HDPE releases absorbed residue slowly, over months rather than days. That delay is the hazard: a surfactant-history tank does not fail on day one, it fails after the system is stocked, cycled and planted, by which point nobody suspects the tank.
A customer wrote to us two years ago about a system that had died, and then generously agreed we could publish what happened. It is the clearest illustration we have of why we refuse certain sales, and I have quoted it in probably forty emails since.
The details are changed slightly at their request. The physics is not.
What they built
A two-tote aquaponics system, built properly. One 275 gallon tote as the fish tank, one top-cut as a media bed on a stand above it. Pump up, gravity down, a bell siphon flooding and draining the bed, washed gravel media, supplementary aeration in the tank.
They cycled it correctly: five weeks with no fish, dosing an ammonia source, watching ammonia rise and fall, nitrite rise and fall, nitrate climb. When ammonia and nitrite read near zero they stocked thirty small tilapia and planted the bed.
Everything in that paragraph is textbook. More careful than most first builds we hear about.
What they bought
Two technical-grade totes from a seller near them, at about $90 each. The seller described them as "clean, previously held cleaning product, washed out".
That description was accurate and honest. The totes had held an industrial surfactant. They had been washed. They were clean.
What happened
Weeks one to four: nothing. Fish fed, plants established, water parameters good. Ammonia zero, nitrite zero, nitrate climbing normally, pH stable.
Week five: fish off their feed. Some flashing — rubbing against surfaces. Water tested fine on every parameter they had a kit for, because the kit tested ammonia, nitrite, nitrate and pH, and the problem was none of those.
Week six: a mass loss overnight. Twenty-three of thirty fish.
They tested everything again. Still fine. They changed half the water, which helped for about ten days, and then the symptoms returned.
Why the delay is the dangerous part
High-density polyethylene is semi-permeable. Over months of service, molecules from the stored product diffuse into the polymer wall. A wash cleans the surface and cannot touch what is in the bulk.
Then, slowly, it comes back out. The gradient runs the other way once the tank is full of clean water, and the wall releases what it absorbed over a period of months — not hours.
Surfactants matter to fish at very low concentrations. They disrupt gill function and the mucus layer, which is exactly what flashing and going off feed look like. And they do not show up on an ammonia-nitrite-nitrate-pH test kit, so every measurement a careful hobbyist takes comes back normal.
So the failure sequence is: fill, stock, cycle, plant, succeed for a month, then lose the stock while all your instruments say the water is fine. By week six nobody is still suspecting the tank — they are suspecting the feed, the pump, the weather, themselves.
A tank that killed fish on day two would be a cheap lesson. One that kills them in week six is an expensive one, and it is the reason we will not sell into this application.
What the half water change tells you
The temporary improvement after a 50% change is the diagnostic. It dilutes the concentration, the fish recover, and then the wall continues releasing and the concentration climbs back. A problem in the water gets fixed by changing the water. A problem in the vessel comes back.
If you are ever in this position, that is the test: does a large water change fix it permanently, or temporarily?
What we will and will not sell
- Aquaponics, stocked aquatic systems, hydroponics with edible crops: documented food-grade with a wash certificate, or rebottled with a new virgin bladder. Nothing else, at any price.
- Non-potable irrigation: technical grade is fine. Plants in soil are far more tolerant than fish in water.
- Drinking water: documented food grade or rebottled, plus whatever your jurisdiction requires.
- Industrial fluids: whatever grade suits the condition requirement. Permeability is irrelevant here.
The gap between the first and second lines on that list is about $190 a unit. It is also the gap between a system that works and a stock loss, and the customer in this story paid $180 to save $380.
The honest footnote
The seller did nothing wrong. They described the totes accurately. The buyer asked the right questions about cleanliness and got truthful answers.
The information that was missing was not about cleanliness at all — it was that cleanliness is the wrong question for this application. Which is why we spend so much of our writing on the difference between a clean tote and a documented one.
783 words · published 29 October 2024 by Dale Prichard, Senior grader at IBC Tote Market LLC. We correct posts rather than quietly deleting them; if something here is wrong, tell us and we will say what changed.