The pH Problem: Why Aquarium Advice Contradicts Itself

Pond Life Aquatics | Science Series | pondlifeaquatics.co.uk

Two experienced keepers. Same hobby, twenty years each. One says the only filter a planted tank needs is the plants. The other says a biological filter is non-negotiable. They aren't arguing about preference — they're arguing about whether a basic principle of fishkeeping is even true.

They're both right. And the reason is pH.

Under one roof here we keep fish from five continents, and the moment you do that you have to accept something the louder voices in the hobby won't: there is no single correct way to keep a fish. In this video we wanted to go after the variable sitting underneath almost every fishkeeping argument online — the one that quietly makes two honest keepers contradict each other.

pH Is the Master Variable

pH isn't just a number on a test strip. Think of it as the cooking temperature of your tank. Some dishes need low, slow heat; others need a screaming-hot wok. Neither is wrong — but some dishes simply won't work at the other setting. Try eggs in a pressure cooker and let me know how you get on.

pH decides whether the ammonia your fish excrete is toxic or harmless. Whether your biological filter runs at full tilt or barely ticks over. Whether your plants outcompete your filter for nitrogen. Whether the iron in your fertiliser feeds your plants or rusts into the substrate. And whether your fish are physiologically at home, or fighting their own bloodstream around the clock.

Change pH and you don't move one of those. You move all five — and because pH is locked together with hardness, alkalinity and dissolved organics, it arrives as a package deal, not a dial you get to turn on its own.

The US EPA describes pH as one of the most important environmental factors limiting species distribution in aquatic habitats (USEPA, CADDIS Volume 2: pH).

The Ammonia Shift Your Test Kit Hides

Ammonia doesn't exist in your water as one substance. It exists as two: NH₃, free ammonia, which is acutely toxic, and NH₄⁺, ammonium, which is roughly a hundred times less so. Which side the balance tips to is decided almost entirely by pH — and it shifts roughly ten-fold per pH unit.

So take two tanks, one at pH 6.5 and one at pH 8.5, both reading the same total ammonia on the same test kit. Identical number, identical colour in the tube. They are not in the same situation. The low-pH tank has its ammonia locked away as harmless ammonium. The high-pH tank has a real slice of that reading sitting there as free NH₃.

This is why one keeper says a quarter of a ppm is fine and another says it'll kill your fish by morning. Both are right — about their tank. The number is the same; the temperature it's cooking at is not. It also explains why blackwater species like cardinal tetras and Apistogramma are so intolerant of ammonia: their native water almost never presented them with the toxic form, so they never evolved a defence against it.

Emerson, K. et al. (1975). Journal of the Fisheries Research Board of Canada — the foundational paper for aqueous ammonia equilibrium calculations, still the global aquaculture standard.

Erickson, R.J. (1985). Water Research — the NH₃:NH₄⁺ ratio in freshwater rises roughly ten-fold per unit increase in pH.

Souza-Bastos, L.R., Val, A.L. & Wood, C.M. (2017). Hydrobiologia — NH₃ toxicity measured across 11 Amazonian species including Paracheirodon axelrodi; blackwater natives are particularly sensitive to NH₃ at elevated pH.

Your Filter Might Not Be the Filter You Think

The classic nitrifying bacteria are fussy about pH too. Nitrosomonas and the nitrite-oxidisers Nitrobacter and Nitrospira want somewhere around pH 7.0–8.0. Drop toward 6.0 and conventional nitrification largely grinds to a halt.

Except blackwater tanks cycle perfectly well. Because "the bacteria stop below 6" is only true of the bacteria you think are running your tank. Nitrotoga is actually stimulated by slightly acidic water. Ammonia-oxidising archaea (Thaumarchaeota) keep working down around pH 5. In extreme engineered systems, Nitrosococcus-related lineages have been documented oxidising ammonia at pH 2.2.

And these aren't rare exceptions. In established freshwater filters, archaea and a group called comammox are the dominant ammonia oxidisers — not the bottled bacteria you poured in on day one. The pH 6 floor is real for starter bacteria. It is not what your mature filter is running on.

Hochheimer, J.N. & Wheaton, F. (1998); Antoniou, P. et al. (1990) — peak biofilter nitrification lies between pH 7.5–9.0. Villaverde, S. et al. (1997) found a 13% increase in nitrification efficiency per unit pH rise across pH 5–9. Brunty, J.L. (1995) tested four biofilter designs: nitrification slowed significantly or stopped entirely below pH 6.0.

Hüpeden, J. et al. (2016); Wegen, S. et al. (2019), Applied & Environmental Microbiology — Nitrotoga sustains nitrification at slightly acidic pH. Fumasoli, A. et al. (2017) — Nitrosococcus-related AOB nitrifying down to pH 2.2.

Sauder, L.A. et al. (2011), PLoS ONE; Bagchi, S. et al. (2014), PLoS ONE; McKnight, M.M. & Neufeld, J.D. (2024), Appl. Environ. Microbiol. — Thaumarchaeota and comammox Nitrospira dominate established freshwater aquarium biofilters, and both are considerably more acid-tolerant than conventional AOB.

So in a low-pH planted tank, plants and acid-tolerant archaea split the nitrogen work — and it barely matters that conventional nitrification has stalled, because most of the ammonia is already ammonium, which is exactly what plants prefer to take up. In a high-pH bare tank, the microbes carry it. Same job, different organisms.

Jampeetong, A. & Brix, H. (2009), Aquatic Botany — in Salvinia natans, maximum uptake rate for NH₄⁺ measured 6–14× higher than for NO₃⁻. See also Ferguson, A.R. & Bollard, E.G. (1969), Planta, and Walstad, D., Ecology of the Planted Aquarium.

Why the "Walstad Works / Walstad Doesn't" Fight Exists

The heavily planted, low-filtration soil tank produces jaw-dropping results for one keeper and a crashed-out mess for the next. Then they argue about who did it wrong.

Neither did. And it isn't really about pH — it's about buffering. A soil substrate constantly produces organic acids. If your water carries a decent carbonate buffer (decent KH), those acids have something to push against and the system holds station. Run the identical setup in ultra-soft, low-KH water and there's no shock absorber; sooner or later the pH falls out from under the tank.

It runs backwards too. Take the soft-water aquascaping playbook into London tap — 300-plus mg/L of dissolved rock — stock the delicate species it's famous for without high-tech CO₂, and they don't thrive. They slowly melt. The method didn't fail either keeper. It was the wrong tool for the water they had.

The Iron Trap

Iron won't stay dissolved in oxygenated water — it oxidises and drops into the substrate where plants can't reach it. So fertilisers wrap it in a chelator to hold it in solution. How long that chelator survives is set entirely by pH:

Chelator	Holds iron to about	Typically found in
EDTA	pH 6.5	budget fertilisers
DTPA	pH 7.5	mid-range fertilisers
EDDHA	pH 9	industrial-strength options

Kasozi, N. et al. (2019), Aquaculture Reports — kinetics of iron oxidation in aerobic, neutral-pH aquatic environments. Chelator stability constants — EDTA (<6.5), DTPA (<7.5), EDDHA (<9.0), gluconate (<6.0) — are standard horticultural and aquatic data.

This is why Takashi Amano's iron-forward ADA philosophy worked so well — he was in soft Japanese tap water. Pour that same approach into London tap and you get orange gunk in the substrate and yellow plants. Someone in London doses a budget iron fert and gets nothing; someone in Manchester doses the identical bottle and gets an explosion of growth. Then they meet in a comment section. Same product, same dose, opposite worlds.

It isn't a hard on-off switch — root exudates and soil acids keep pockets of iron mobile even in hard water. But as a working rule, on hard alkaline water an EDTA fertiliser is the wrong tool. Use DTPA or EDDHA.

Tolerance Isn't the Same as Thriving

"My fish are fine at that pH." I believe them — but fine usually means not dead, and that's a very low bar for something you chose to keep.

Every fish has two ranges. The one where it thrives, doing effortlessly what it evolved to do, and a wider one where it merely copes — surviving, but paying for it. From the outside they look almost identical. A coping fish still swims and still eats. It just never quite colours up, never spawns, never settles, and ages faster than it should.

Cardinal tetras held at pH 6.0, then 4.0, then 3.5 for five days showed no change in body sodium at all — not stressed, at home. In the same study, four other Rio Negro natives dropped to pH 3.5 lost salts only mildly, while a farm-raised generalist — tambaqui, held and tested under identical conditions — lost them seven times faster. Exceptional acid tolerance turns out to be a general characteristic of fish from that river, and a farmed outsider simply doesn't have it.

That runs both directions. Put a cardinal built for pH 4 into hard tap at pH 8 and it won't die — it'll cope, doing constant invisible work to hold its ion balance, for the rest of a life that's probably shorter than it should be. It tolerated the pH. It never thrived at it.

This is why, for soft-water species, we lean toward captive-bred stock wherever we can. Not as a compromise — as the honest choice. Generations bred in harder water have genuinely shifted what "home" means. It isn't an iron rule (red rasboras, Boraras maculatus, are almost all wild-caught and surprisingly robust), but it's the safer default.

Gonzalez, R.J. et al. (1998). Effects of water pH and calcium concentration on ion balance in fish of the Rio Negro, Amazon. Physiological Zoology, 71(1), 15–22. DOI (via PubMed)

The Hard Limits

None of this means anything goes. Nuance is not the same as no rules, and there are limits you cannot clever-talk your way past.

You cannot keep a wild Parosphromenus in pH 8 tap water. You cannot keep a Tanganyikan cichlid in soft acidic blackwater. You cannot run a heavily stocked tank with no filter and no plants and expect the pH conversation to save you.

And the one that overrides everything: free ammonia above roughly 0.02 mg/L is dangerous to most fish. That isn't regional and it isn't negotiable. All pH does is decide how much of your total reading sits in that toxic form. pH sets the dose. It does not repeal the limit.

USEPA (2013), Aquatic Life Ambient Water Quality Criteria for Ammonia — Freshwater. The generally accepted aquaculture threshold is un-ionised NH₃ above roughly 0.02 mg/L, varying by species and life stage.

What This Means in Practice

Almost all fishkeeping advice reaches you with its chemistry stripped off. Someone in Bristol on pH 6.8 tap tells you exactly what's worked for fifteen years, and they aren't lying. Someone in Edgware on hard chalk-aquifer water at pH 8 follows it to the letter and watches it fall apart. Both assume the method was the variable. It almost never is. The variable was in the tap before either of them filled a tank — and because it's their water, it's invisible to them.

So the question isn't "what's the correct method?" It's "what's the correct method for the water I actually have?"

For us, that means most of our stock runs on straight tap and we choose what to stock around the water we've got. Rift Lake cichlids and Australian rainbowfish don't find London tap a compromise — it's practically a love letter. What we don't do is fight it: no wild altum angels, no wild discus, no liquorice gouramis, however much I'd like to.

And for the fish that genuinely need soft water, botanicals aren't the answer. Tannins will tint your water and nudge the pH a touch. They will not touch 300 mg/L of carbonate hardness — the buffer swallows the acid and holds the line. For real soft water you run RO and remineralise back up to what the fish actually wants. That isn't fussiness; it's what the chemistry demands.

Match the fish to your water, or build the water to match the fish. What you don't get to do is drop a blackwater specialist into liquid rock, watch it survive, and call that success.

And if you're in a hard-water area and don't fancy plumbing in a full RO unit, one genuine recommendation: Spotless Water — pre-purified, ready to go. We're not sponsored (we'd happily take the call), but that link carries our reference code. It gets you a few free fills and throws a little back to us for running the shop.

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