If you’ve ever kept an aquaponic system going, you’ve met the pH problem. You fill the system, the water is fine. Two weeks later, the pH has drifted down. You add a handful of crushed shell, or a spoon of agricultural lime, or a commercial buffer. The pH comes back up. A fortnight later, it’s falling again. You do it again. Then again. And so on, forever, because the system is quietly producing acid every day and you are the only thing stopping it from crashing.
iAVs doesn’t do that.
You can run an iAVs for months without touching the pH. The water settles into roughly 6.4, sits there, and stays there. No lime. No buffers. No shell. The first time you notice you haven’t added anything in a while, it feels vaguely suspicious. It isn’t. It’s the system working the way it was designed to work.
This article explains why.
What’s actually acidifying the water
In any recirculating fish‑and‑plant system, the bacteria that convert fish ammonia into plant‑usable nitrate do so by producing acid. For every ammonium ion that gets oxidised into nitrate, two protons – hydrogen ions – are released into the water. That’s the acid. It is unavoidable; it is the same process happening in every aquaponic and every iAVs system on Earth. Nitrification and acidification are the same event, seen from two angles.
In a conventional aquaponic system, that acid has nowhere to go, and the pH drifts down until you step in. That is why lime, shells and buffers are a permanent fixture of aquaponic operating routines. It’s also why a lot of aquaponic literature talks about “base additions” as though they were a normal part of system maintenance – because in those systems, they are.
What the plants do in iAVs
In iAVs, the same acid is being produced – the nitrogen chemistry is the same – but something else is going on as well. The plants are drinking it off the water.
When a plant root takes up a nitrate ion, the root gives back an OH⁻ or an HCO₃⁻ – an alkalising ion – to balance the electrical charge. One molecule of nitrate taken up is one molecule of alkalinity put back. That is standard plant physiology; it is described in any textbook of plant mineral nutrition (Marschner, 2012). What makes iAVs different is not the mechanism. It is the scale. Because there is no separate mechanical or biological filter between the fish tank and the plants, and because the crop in an iAVs sand bed is not a thin row of lettuce at the side of the tank but a full cropping bed at the full working ratio (6 m² of bed for every 1,000 L of fish tank), the plants are taking up nitrate fast enough and in quantities large enough to cancel the acid from nitrification as it is produced.
The system doesn’t “resist” acidification. It neutralises it in real time.
The experiment that proved it
This is not theory. McMurtry and colleagues showed it directly in 1997. In one of their trial runs, they deliberately pulled the plants out of the biofilters and left the sand bed, the fish, the feed, and the bacteria exactly as they had been before. Within 42 days, the water pH fell from about 6.0 to 4.3. Then they put the plants back. The pH climbed back up.
That’s the whole argument in one experiment. When the plants are there, pH sits at 6.0–6.4 without intervention. When the plants are gone, pH crashes. The buffer is the crop.
Why this matters to a fish keeper
A stable, mildly acidic pH is not just convenient. It is the condition fish welfare actually requires.
Fish are harmed by free ammonia, which is the gas form (NH₃), not the ammonium ion (NH₄⁺). The water pH controls how much of the total ammonia pool is in which form. At pH 6.4, about 99.9% of the ammonia in the water is in the safe ammonium form and only 0.1% is in the toxic gas form. Push the pH up to 8.0 – a fairly typical tap‑water value – and that toxic fraction is suddenly thirty to forty times higher. iAVs doesn’t just “have a nice stable pH.” It parks the pH in exactly the band where the fish are safest without anyone having to aim for it.
Meanwhile, the same pH band happens to be the one at which most vegetable crops take up phosphorus, iron, manganese and zinc most efficiently. Both sides of the co‑culture are being kept in their own sweet spot by the same mechanism.
What this replaces
In an aquaponic system, keeping pH in a healthy band typically means: a supply of shell or lime, a commercial buffering salt (usually potassium bicarbonate or calcium hydroxide), a pH meter used often enough to catch a drift before it becomes a crash, and a plan for what to do if the drift becomes a crash anyway.
In iAVs none of those are operating expenses. You still own a pH meter, because a meter tells you when something is wrong. What you don’t own is the shopping list.
The two misreadings worth clearing up
Two ideas tend to arrive at iAVs from the aquaponics literature and confuse newcomers, and they’re worth naming because they both sound right and both mislead.
“pH is held at 6.4, so the nitrogen must be in ammonium form.” The pH does control the ammonium/ammonia equilibrium (NH₄⁺ vs NH₃, the one relevant to fish toxicity), and at 6.4 the nitrogen in that pool is virtually all NH₄⁺. It does not control the bigger picture – the split between ammonium nitrogen and nitrate nitrogen in the water overall. That split is set by the nitrifying bacteria in the sand bed, and in a mature iAVs it is roughly 20 to 100 times more nitrate than ammonium. The plants see mostly nitrate, and it is that nitrate uptake that does the buffering.
“If the pH is stable, it must be denitrification.” In some recirculating aquaculture designs, pH is held up by anaerobic bacteria that convert nitrate into nitrogen gas, a process that consumes acid. That is not what is happening in an iAVs sand bed. The sand bed is kept aerobic by the reciprocating irrigation cycle, dissolved oxygen in the water is high, the nitrate in the water accumulates rather than disappearing, and – as the plants‑out experiment above shows – removing the plants crashes the pH. If denitrification were doing the buffering, removing the plants would do nothing to the pH, because the bacteria would still be there. The pH crash is the fingerprint of a plant‑driven buffer.
The takeaway
iAVs doesn’t “have good pH stability” as a lucky accident. It has good pH stability because the crop is doing the chemical work that aquaponic operators normally have to do with a lime bag. Stop feeding the fish, and the chemistry stops. Pull the plants out, and the chemistry stops. Leave the system assembled and running at its design ratios, and the chemistry runs itself.
This is one of the reasons the system is cheap to operate, simple to teach, and forgiving of inattention: you don’t have to remember to do something that the system does for you every day.
For the numbers and the operating ratios, see the iAVs Handbook*. The primary research this article draws on is McMurtry et al. (1997), Effects of biofilter/culture tank volume ratios on productivity of a recirculating fish/vegetable co‑culture system.*