Why driftwood turns the water brown and lowers pH

Written 2026-11-04

Tannins (humic acids) leaching from driftwood tint the water brown and pull pH down on their own, separately from KH (buffering capacity). Running the Water Kiwoogi simulator on a 60cm tank shows that adding two pieces of regular driftwood drops the pH from 7.67 to 7.27 the same day, and three pieces take it to 7.06 — tannin lowers pH by 0.20 per unit, capped at 1.10, so piling on driftwood can’t push pH down forever.

Tannin pulls pH down directly, separately from KH

In this simulator, pH is normally set by KH and CO₂, and tannin shaves a further amount off that number. Each piece of regular driftwood releases 1.0 units of tannin, and every unit of tannin pulls pH down by 0.20. So a 60cm tank filled with plain tap water starts at pH 7.67, but the same tank with two pieces of driftwood drops to 7.27 the day it’s added, and three pieces take it to 7.06. The gap shows up on day one, before KH has moved at all — tannin lowers pH directly, ahead of anything it does to KH. That drop is capped at 1.10, so stacking driftwood (around six pieces, once tannin reaches 5.5) won’t push pH any lower than that.

Tannin also wears KH down slowly, but it won’t go below 1.2

Besides lowering pH directly, regular driftwood also eats away at KH at about 0.03 a day. But that drain has a floor — once KH hits 1.2, driftwood can’t pull it down any further. Running a fishless tank with driftwood alone for 90 days, two pieces bring KH from 4.0 down to 2.48 and pH settles at 7.06; three pieces bring KH to 1.72 and pH to 6.70. Neither case gets anywhere near the crash point, where KH under 0.25 lets pH collapse to 5.0. The KH that bacteria consume while processing fish waste has no such floor and can keep dropping without end if you skip water changes — but the share driftwood tannin eats stops at 1.2.

It helps soft-water fish, but dumping in too much at once backfires

Cardinal tetras need soft, slightly acidic water — pH 4.5 to 7.0, ideally 6.2. Stock ten of them into a plain-tap-water tank at pH 7.67 and six die over 90 days, leaving four. Add two pieces of driftwood to bring the pH down to 7.27 and nine survive. But push the pH even lower, to 7.06, with three pieces of driftwood, and only seven survive — a lower, more suitable pH, and a worse outcome.

The driftwood itself is the cause. New driftwood grows white fungus on its surface for its first two weeks, releasing organic waste the whole time, and that load scales with how many pieces you add. With two pieces, nitrite peaks at 1.58ppm on the test kit; with three, it climbs to 2.42ppm. That extra nitrogen load lands right when the bacteria colony is still young, and it costs two more fish than the two-piece setup — those deaths cluster between day 7 (the first death) and day 9 (the last). Driftwood is great for pushing pH down, but until a tank finishes cycling, the piece count is a variable to watch as carefully as stocking density. If you’re unsure, it’s worth checking with a fish store or an aquarium specialist.

The button below runs the tank from this guide — a 60cm standard tank, two pieces of regular driftwood, ten cardinal tetras. Bump it to three pieces and you can see for yourself how a lower pH comes with a rougher nitrite spike early on.

The numbers in this guide come from the Water Kiwoogi simulator model; real tanks can differ.