The kinds of water aquarium fish come from

A species page can tell you a temperature and a pH. What it usually cannot tell you is which of those numbers is load-bearing, and that is decided by the habitat rather than by the animal. A fish from a blackwater tributary is adapted to water with essentially no dissolved mineral in it; the acidity is a consequence of that, not the requirement. A fish from a hill stream is adapted to oxygen and turbulence, and its pH is whatever the local rock happens to produce.

Get that the wrong way round and you produce the two commonest failures in the hobby: chasing a pH figure for a fish that needed soft water, and providing perfect chemistry for a fish that needed current.

AquariumHQ assigns every organism in the catalogue to one habitat class and checks the published water band against what that kind of water is actually like. Where the two disagree, something is wrong and the build fails. Where they agree, the record can say why its numbers are what they are.

Blackwater is the extreme. Tannin-stained water draining podzolic forest sand, carrying almost no dissolved mineral: conductivity in the low tens of microsiemens, pH commonly between 4 and 5, hardness below the resolution of a test kit. The Rio Negro is the type example. The colour is not the point — the absence of mineral is, and it means the water has almost no buffering and a pH that moves fast.

Peat swamp forest goes further still. Standing water over deep peat in Sundaland, with a pH floor near 3 and an oxygen regime that would kill most fish, which is why so many of the animals from it breathe air. Licorice gouramis, chocolate gouramis and most of the wild Betta species come from here.

Soft forest stream is the commonest class in the trade and the one the conventional community-tank setting actually fits: shaded, mineral-poor running water at pH 5.5–7 and 1–6 dGH. Shaded lowland forest streams in the Amazon, the Guianas, West Africa and Southeast Asia converge on this chemistry because the geology and the canopy do the same work in each. That convergence is real, and it is why so many unrelated families share a band.

Soft-water lakes — Mai-Ndombe, blackwater lagoons — have blackwater chemistry without blackwater current, and the animals from them generally want the softness and specifically do not want flow.

Limnologists divide Amazonian water into black, clear and white, and the division predates anybody's interest in keeping the fish. Black water is the leached-sand case above. Clearwater rivers — the Tapajós and Xingu type — drain ancient shield geology, are transparent, mineral-poor and well oxygenated, and run pH 6–7.5 at 1–8 dGH. White water carries Andean sediment and is not soft at all: near-neutral, 2–10 dGH, appreciably more dissolved mineral, turbid enough to hide a fish at arm's length.

Species from the white-water channels are sold as though they wanted Rio Negro conditions and they do not. The distinction matters most for the corydoras, several of which come from white-water margins and are given blackwater treatment because their genus is associated with it.

Hard alkaline water comes from carbonate and volcanic geology and travels across continents wherever that geology occurs — Central American rivers, karst springs, hard-water lakes. pH 7.4–8.5, 10–25 dGH. The commonest failure with these species is a keeper softening water that did not need softening.

The African rift lakes are usually treated as one product and are three different waters. Lake Tanganyika is the most alkaline water any commonly kept aquarium fish comes from: pH 8.6–9.2, conductivity around 600 µS/cm, and a temperature so stable that the lake is its own thermostat. Lake Malawi is roughly a third of Tanganyika's conductivity — alkaline, at pH 7.7–8.6, but only moderately hard at about 5–11 dGH. Lake Victoria is shallower and younger again, at 90–160 µS/cm, softer than either.

One rift-lake salt mix for all three is the standard practice and it overshoots two of them. AquariumHQ's own catalogue was found doing a version of the same thing during this sprint: several Malawi cichlids carry hardness bands above what the lake runs at, and the Lake Victoria cichlid's band is nearer Malawi's than Victoria's. Those values are now recorded as trade conventions rather than as habitat statements.

Rice paddies, ditches and drains are now the primary habitat of several aquarium species, and their temperature and oxygen regime is nothing like the natural habitat those species are usually described as having. Enormous daily and annual swings, low oxygen, shallow sun-warmed water. The medaka, the paradise fish, the zebra danio and the wild Betta splendens are all from managed agricultural water, and several of them specifically do not want to be kept at 26 °C all year.

Seasonal pools are the annual killifishes' habitat and the chemistry is the least important thing about them. What matters is that the animal's whole life is compressed into a few months, and a keeper who slows that down with cool, stable water is often doing the fish a favour.

Semi-terrestrial margins are where the vampire crabs live — damp forest floor beside water, entering it briefly. A water band for one of those animals is a provision rather than a habitat requirement, and what it is actually asking for is land, humidity and somewhere to shed.

And some animals have no wild referent left at all. The goldfish, the koi and the shop guppy have been bred in captivity for so many generations that their wild population's water is interesting history rather than husbandry. The marbled crayfish goes further: it arose in captivity and has no wild origin to refer to.

Every figure AquariumHQ publishes for a habitat class is a generalisation about a KIND OF WATER BODY. It says what Rio Negro blackwater is like. It says nothing about any particular fish that lives in it and must never be read as though it did.

The ranges are wide because the water is. The Rio Negro is not one conductivity; it is a river whose conductivity varies by site, by season and by how much white water has mixed in by the time you sample it. A confident-looking single number would be false precision wearing a unit that makes it look rigorous.