A mature iAVs sand bed is not just a filter. It is a storehouse. Over months of feeding, organic solids have been deposited into the furrows and mineralised into a living soil, and that soil holds plant-available nutrients long after the fish are gone. The water column is only part of the story.
That is what this video demonstrates. The operator drained the system completely to clean the liner, then refilled it with pure rainwater. No fish in the system. The plants kept growing.
The lesson is buffering capacity. A mature iAVs biofilter stores and releases nutrients across time. That gives the operator flexibility between fish cycles, and it softens the fragility you see in systems that depend entirely on the immediate nutrient concentration dissolved in the water. In iAVs, the nutrients live in the soil, not just in the water passing over the roots.
So you don’t need fish??
You do need fish. In this example the system had been running without interruption for three years, which is what allowed that nutrient reserve to build. The fish are the input. Remove them permanently and the storehouse slowly empties.
But note what “need fish” means in practice. iAVs is roughly 90 percent horticulture. In an ordinary garden you do not apply fertiliser every day. You feed the soil once, maybe once a season, and the soil biology releases nutrients to the plants over time. An established iAVs behaves the same way. Feed input is still the master control over the medium and long term, but the daily number matters less once the bed is mature, because the sand is releasing what it already holds. Keep the feed rate adequate over time and the biology handles the timing.
iAVs is 90% gardening – and in a ‘normal’ outside garden you don’t need to apply fertilizer every day, it’s usually something that gets done once a year – nice and easy! – nature and the soil microbes do the work. In the same way, once an iAVs is established and matured, the feed input per day is something that you don’t need to constantly think, or worry about, as long as the feed rate over time is adequate, the rest takes care of itself.
Is this the standard way to do it?
No. Most operators re-stock the fish as soon as the last batch is harvested. Many commercial operators manage fish so the system stays in continuous production rather than sitting idle between single batches. Running fishless is a deviation from standard practice, and it should be treated as an experiment rather than a routine. It was not part of the documented NCSU research methodology.
Why run without fish?
There are practical reasons. In cold climates the water may be too cool for the fish for part of the year. Rather than shut the system down, you can keep the crops growing through the cold season and add fish when the weather warms.
In the video, the owners were watching how the surrounding trees cast shade, and they wanted to observe the sun’s path for a while before committing to a greenhouse. That pause created the chance to film this experiment.
How do you know there will be enough nutrients?
Without testing, you do not know. That is the honest answer. In this example the operators simply tried it and watched the plants for signs of deficiency. If you want more certainty, test the tissue and the bed, and keep records. What this demonstration shows is that a mature bed can carry plants for a period without fish. It does not establish how long, for which crops, or at what intensity. Those remain open questions.
Any other benefits?
The obvious one is flexibility. The less obvious one is research. A fishless window might let you run specific crops to draw down particular elements that could otherwise accumulate, zinc or copper for example, before the next stocking. That is an interesting idea, but let me be plain: to my knowledge it has not been tested against a control. It is a hypothesis, not a finding.
The point worth repeating is the mechanism. In iAVs the nutrients are not held in the water. They are held in the soil. That is why the system keeps running when the water is emptied and the fish are gone, and it is the same reason the method behaves more like a garden than a hydroponic rig.
