Amazon water in a glass box, and what South American fish actually need

An Amazon or South American aquarium wants soft, slightly acidic, warm water: roughly pH 5.5–7.0, GH 1–6 °dGH, KH 0–3 °dKH, 24–28 °C (discus 28–30 °C), a conductivity somewhere around 50–300 µS/cm, and nitrate kept low, under about 20 mg/L. Tank-bred tetras, corydoras and most dwarf cichlids are happy anywhere in that window, and many tolerate harder water too. Wild-caught fish and anything you want to breed belong at the soft, acidic end. Above all, the water should be steady: a stable pH 6.8 beats a "perfect" 5.8 that wanders.

Parameter	Target for the tank	Notes
pH	5.5–7.0	tank-bred community toward 6.5–7.0, wild fish and breeding 5.5–6.5
GH	1–6 °dGH	breeding tanks at the low end
KH	0–3 °dKH	below 1 °dKH the pH needs watching
Temperature	24–28 °C	discus 28–30 °C
Conductivity	~50–300 µS/cm	wild fish and breeding toward the bottom
NO₃	under ~20 mg/L	discus keepers usually aim for 10 or less

Those are targets for an aquarium, not readings from a river. The Rio Negro runs far softer and more acidic than any of these numbers, and copying it one-to-one in a closed glass box with a filter and a bioload is a project for specialists. The table is the zone where South American fish do well in captivity and the water still forgives a missed water change.

What are the three Amazon water types?

"Amazon water" isn't one thing. Limnologists sort the rivers of the basin into three broad types by their colour, and the colour tells you a lot about the chemistry.

Blackwater — the Rio Negro is the textbook case — drains old, leached sandy soils and flooded forest. It's stained the colour of strong tea by humic and fulvic substances from decaying leaves. The pH typically sits somewhere around 3.5–5.5, the conductivity is often below 30 µS/cm, and there is practically no measurable hardness at all. It is clear but dark, poor in nutrients, and comparatively low in bacteria. Cardinal tetras and many wild Apistogramma come from waters like this.

Clearwater rivers such as the Tapajós and the Xingu drain the ancient rock shields to the north and south of the basin. They're transparent and often greenish, with a pH of roughly 5 to 7 and a very low conductivity as well. Little sediment, little humus, little dissolved mineral.

Whitewater — the Solimões and the Amazon mainstem itself — comes down from the Andes carrying a heavy load of sediment, which makes it turbid and milky-brown rather than white. It's the richest of the three in dissolved minerals, with a pH of roughly 6.5–7.5. "Richer" is relative, though: by the standards of most European tap water, even whitewater is soft.

Near Manaus the black Rio Negro and the brown Solimões run side by side for kilometres before they mix. For your tank the point is simple: "South American" covers a wide spread of water, and a whitewater species won't thank you for Rio Negro chemistry.

Do your fish actually need soft water?

Usually less than the internet suggests, and sometimes more.

Most of what's sold as South American community fish is tank-bred, often for dozens of generations. Neon tetras, the common corydoras, angelfish, farmed Apistogramma colour forms and bristlenose plecos have adapted to whatever water their breeders used, frequently medium-hard. They live long and colour up at pH 7 and 8–10 °dGH. If that's your tap water and you keep bred fish, there's no need to fight it.

Two groups really do need soft water:

• Wild-caught fish. A large share of cardinal tetras, many wild Apistogramma, wild discus and plenty of rarer tetras still come out of blackwater and clearwater rivers. They tolerate harder water to a degree, but rarely look their best in it.
• Anything you want to breed. The eggs of many soft-water species fail to develop in hard water: the fish court, spawn, and nothing hatches. Breeding setups for discus, Apistogramma and most tetras run at the very low end of the table, often with GH around 1–2 °dGH.

So the honest first step is to ask your dealer or breeder what water the fish were raised in. That number is a better guide than the river on the label.

How do you get soft water?

You have two levers: change the source water, and add natural acids and tannins once the source is soft.

RO water is the foundation. A reverse-osmosis unit strips almost everything out of tap water. Don't fill the tank with it straight: water with zero minerals offers fish no stability and plants no calcium or magnesium. Instead, remineralise it for GH only with a GH-raising salt (the kind sold as "GH+" rather than "GH/KH+"), aiming for something like 2–5 °dGH, and leave KH low. Alternatively, cut your tap water with RO to land where you want; a 3:1 RO-to-tap blend leaves roughly a quarter of your tap's hardness. The hardness converter handles the °dGH, ppm and calcium arithmetic, and the GH vs KH guide explains why the two hardnesses can and should be set separately here.

Then add the forest. Three materials have been used for decades:

• Peat in the filter releases humic acids, lowers KH and pH a little and softens mildly. Use it in a mesh bag so you can take it out again.
• Catappa leaves (Indian almond leaves) release tannins, colour the water amber and bring the pH down slightly. They double as cover and grazing.
• Alder cones do the same in a smaller dose: one or two per 20–30 litres is a common starting point.

One thing to know about all three: they only move the pH meaningfully when the KH is already low. In hard water the carbonate buffer simply neutralises the acids, so you get tea-coloured water at pH 7.5. Soften first, then add botanicals. And skip "pH minus" products. They lower pH by burning through KH, which works right up until the buffer is gone and the pH falls off the edge.

Why is a KH near zero a risk?

KH is the buffer that absorbs the acid a tank produces every day: respired CO₂, the acid released by nitrifying bacteria, organic acids from wood and leaves. With KH at 4 or 5 °dKH, that load barely moves the pH. With KH at 0–1 °dKH, there's almost nothing in the way, and the pH can slide a full point over a few days, or drop sharply overnight. The why KH drops guide walks through each drain in detail.

Humic substances add a little buffering of their own, which is why a well-run peat tank can sit at pH 5 for months. But that buffer is weak and unpredictable, and two things make the risk concrete:

• Below roughly pH 6, nitrifying bacteria slow down. Ammonium then accumulates, mostly in its less toxic NH₄⁺ form while the pH stays low. A big water change with harder water can lift the pH and convert part of it to toxic ammonia within hours. If you run a very soft tank, keep the ammonium test in the routine, and keep your change water close to the tank's chemistry.
• Big swings hurt more than wrong numbers. A stable pH 6.5 is easier on fish than a tank that bounces between 5.5 and 7 depending on when you last changed water.

That's why many keepers of tank-bred South American fish hold KH around 1–3 °dKH instead of zero: it buys a lot of margin. Small, regular water changes with prepared water of the same hardness and pH help more than any additive; the water change frequency guide covers the cadence.

What happens with CO₂ injection in soft water?

CO₂ injection lowers the pH further, and in soft water much more steeply: with little KH to buffer the carbonic acid, the same dose of gas moves the pH more. By the standard formula, 20 mg/L of CO₂ sits at about pH 6.8 with KH 4, but at about pH 6.2 with KH 1. Start low, raise the bubble rate slowly, and watch the fish at the end of the photoperiod, when CO₂ peaks.

There's a second catch. The usual way to estimate CO₂ from pH and KH, the method behind the CO₂ calculator and the CO₂ parameter page, assumes carbonic acid is the only acid in the water. In a tank full of tannins and humic acids, that assumption breaks: the pH is lower than CO₂ alone would explain, so the formula overestimates the CO₂. At KH 0–1 a single drop of the KH test also changes the result dramatically. In a blackwater tank, treat the calculated CO₂ as a rough hint at best and watch the fish and plants instead.

How do you measure pH and conductivity in very soft water?

At very low KH, test methods start to misbehave.

pH: use a calibrated meter. In nearly unbuffered water the indicator dye of a drop test can nudge the sample's pH itself, and the colour steps are too coarse to separate 5.8 from 6.2. A meter is the better tool, with three habits: calibrate it regularly against 7.00 and 4.01 buffer solutions, give the reading noticeably longer to settle (probes react more slowly in very soft water), and measure at the same time of day each time.

Conductivity is the soft-water keeper's GH shortcut. A conductivity meter sums all dissolved ions in µS/cm, cheaply and in seconds. It doesn't say which ions, so it doesn't replace a GH test, but it's ideal for checking a batch of RO water or a mix before it goes in. As a rough rule of thumb for plain mineral water, each degree of German hardness adds something around 30 µS/cm; fertilisers, salt and nitrate push the reading up without adding hardness. Tap water in many regions reads several hundred µS/cm, fresh RO water typically single digits to low tens.

Keeping track of it in reefnotes

In a soft-water tank the slow lines matter: a KH that drifts from 2 to 0.5 over three weeks, a GH that creeps up because you top off with tap, a pH that wanders after new leaves go in. You only see those drifts if you write the numbers down. reefnotes is a free logbook for exactly that. You log pH, KH, GH, calcium and nitrate, and the trend chart shows where they're heading.

One setting is worth making right away. The app's default freshwater bands are tuned for ordinary community tanks: pH 6.0–7.5, calcium 20–80 mg/L. A deliberately soft tank would be flagged as "too low" all the time. So set your own OK ranges for that aquarium: something like pH 5.5–6.5, calcium below 20 mg/L, temperature to suit your fish. The "Discus / Amazonian softwater" preset is a good starting point that you then adjust. From then on reefnotes measures your tank against what it's meant to be, not against the average community tank.

The rest is the long view: pick a target, prepare your water the same way every time, and change one thing at a time. South American fish don't need river water. They need water that's soft enough and doesn't change its mind.