Guide 02 · Water Chemistry

The chemistry that keeps the water alive

You are not really keeping fish. You are keeping water, in a condition that keeps fish. Almost everything that goes wrong in an aquarium is a chemistry problem first and a fish problem second. Here is the chemistry.

Reading time ~11 minutes · The science beneath the Complete Method

The nitrogen cycle: the engine of the whole hobby

Fish excrete ammonia, and so does decaying food and waste. Ammonia is acutely toxic. In a healthy, established aquarium it never accumulates, because two groups of bacteria continuously convert it: one group oxidises ammonia to nitrite, and a second oxidises nitrite to nitrate.^[2] This sequence — ammonia → nitrite → nitrate — is the nitrogen cycle, and it is the single most important thing to understand in fishkeeping.

Figure 1 — The nitrogen cycle. The two bacterial steps detoxify waste; only the water change removes the final product.

A corrected piece of folklore

For decades hobby literature credited the genus Nitrobacter with the nitrite→nitrate step. Molecular studies of real freshwater aquaria found this was wrong: the dominant nitrite oxidisers are Nitrospira-like bacteria, with classic Nitrobacter largely absent.^[1]^[3] It changes none of your actions, but it is a good example of the Institute's rule: cite the evidence, not the received wisdom.

Cycling a new tank

A brand-new tank has no bacterial colony, so it cannot process ammonia. "Cycling" is the weeks-long process of growing that colony before the tank is fully stocked. Ammonia is introduced — ideally without animals, via a bottled ammonia source or a small amount of fish food — and you test daily. First ammonia rises and falls; then nitrite rises and falls as the second bacterial group establishes; finally you read zero ammonia, zero nitrite, and some nitrate. Only then is the tank cycled.^[2]

Why "fish-in" cycling is cruel

Cycling with fish already in the tank means exposing them to the very ammonia and nitrite spikes the cycle produces — both demonstrably toxic.^[4]^[6] A fishless cycle grows the same colony without that cost.

Ammonia: why the target is zero

Ammonia in water exists in two forms in equilibrium: ionised ammonium (NH₄⁺), which is relatively mild, and un-ionised ammonia (NH₃), which is highly toxic and crosses fish gills readily. The balance between them is governed by pH and temperature — as pH and temperature rise, the toxic NH₃ fraction increases sharply.^[4] This is why the same total ammonia reading is more dangerous in a warm, alkaline tank than a cool, acidic one. Government water-quality criteria set ammonia limits precisely because of this toxicity to aquatic life.^[5] For the aquarist the rule collapses to something simple: any detectable ammonia is too much.

Nitrite: the second poison

Nitrite is the intermediate product, and it is also toxic. Absorbed across the gills, it interferes with the blood's ability to carry oxygen — the condition known in aquaculture as "brown blood disease" (methaemoglobinaemia). Usefully, chloride in the water competes with nitrite uptake and is protective, which is one reason a little aquarium salt is sometimes used during a nitrite problem.^[6] As with ammonia, the target in a stocked tank is zero.

Nitrate: the manageable end-product

Nitrate is the end of the line — far less toxic than ammonia or nitrite, but not harmless, and it accumulates because the cycle has no further biological step to remove it in a typical aquarium.^[7] This is the whole reason the partial water change exists: dilution is the practical means of keeping nitrate from climbing without limit. Live plants consume some nitrate as fertiliser, which is why heavily planted tanks often need less frequent changes — but for most systems, the water change is the control.

The stability triangle: pH, KH and GH

The three measurements below are not about toxicity; they are about stability. Fish handle a surprisingly wide range of values, but they handle sudden change very poorly. These three are interlinked, and understanding the link is what prevents the most baffling aquarium crises.

The three stability parameters
Measure	What it is	Why it matters
pH	How acidic or alkaline the water is	Most fish tolerate a broad band; the danger is rapid swings, not the absolute value.^[14]
KH (carbonate hardness / alkalinity)	The water's buffering capacity — its resistance to pH change	Adequate KH holds pH steady; low KH lets pH crash unpredictably.^[15]
GH (general hardness)	Dissolved calcium and magnesium	Affects fish osmoregulation and which species thrive; relevant to breeding.^[15]

The crucial relationship is between KH and pH. KH is the buffer: it neutralises the acids that naturally build up in an aquarium (from fish respiration and biological processes) and so keeps pH from falling.^[9] When KH is exhausted, pH can drop suddenly and steeply — an event called a "pH crash." The fix is rarely to chase pH directly; it is to maintain KH, and, as ever, to change water regularly so the buffer is replenished and acids are removed.

The practical takeaway

Do not try to hit a "perfect" pH from a chart. Choose fish suited to your tap water's natural values, keep KH high enough to buffer, and let consistency do the work. A stable, ordinary tank beats a chemically "ideal" but volatile one every time.^[14]