Tannins and Humic Substances in Aquarium Water

The chemistry of tannins and humic substances from leaves, wood and peat, how they colour water, lower pH and absorb light, what research says on health effects, and removal with carbon.

Chemistry: two overlapping groups

Tannins are astringent polyphenolic molecules made by plants that bind and precipitate proteins; to act as protein binders they need at least 12 hydroxyl groups and five phenyl groups. They fall into hydrolysable and condensed tannins, and condensed tannins can make up as much as half of the dry weight of leaves. Tannins are found in the leaves, bark and wood of most plant families.

Humic substances are coloured organic compounds that form slowly as microbes break down dead plant matter, including lignin and cellulose, and they resist further decay. They are divided by solubility: humic acid dissolves at neutral and alkaline pH but precipitates below pH 2; fulvic acid dissolves at any pH and has more carboxylic groups and a lower molecular weight; humin is insoluble at all pH values. Aquatic humic acids are reported at molecular weights up to about 1,700 and fulvic acids below 900. Their carboxylic and phenolic groups make them weak acids and allow them to chelate metal ions such as calcium, magnesium and iron, which changes how available those metals are to organisms. Together these compounds form most of the dissolved organic matter in fresh water, and the coloured fraction is called coloured (or chromophoric) dissolved organic matter, CDOM.

Sources in the aquarium

• Wood: driftwood that has not been prepared leaches tannins that lower pH and tint the water the colour of tea; pale hardwoods contain less tannin than dark woods.
• Leaves: leaf litter is rich in tannins and other phenolics. Dried leaves of Terminalia catappa (Indian almond) are traditionally added to water by fish farmers and by keepers of Betta splendens; extracts contain hydrolysable tannins, phenolic acids, flavonoids and organic acids.
• Peat: used in freshwater aquaria, peat softens water by acting as an ion exchanger and stains it yellow to brown as tannins leach out. Sphagnum moss, a main peat former, itself secretes tannins.

In nature the same compounds create blackwater rivers such as the Rio Negro, where tannins and humic substances leach from forest and swamp vegetation. Wikipedia reports a mean pH of about 5.1 for blackwater rivers versus about 6.9 for whitewater rivers, conductivity of about 9 μS/cm, similar to rainwater, and few dissolved minerals; snails are scarce there because the water holds too little calcium for shells.

Effects on pH and KH

Because humic and fulvic acids and leached tannins are acids, they lower pH, and peat additionally removes hardness by ion exchange. How far pH falls depends on alkalinity (KH). The SRAC fact sheet on pH and alkalinity explains that alkalinity, mainly bicarbonate and carbonate, is the water's capacity to absorb acid before its pH changes: in moderately to highly alkaline water, more acid is needed to shift pH, whereas in poorly buffered water small acid inputs move pH a long way. In practice, tannin sources darken hard, alkaline water but change its pH little, while in soft water with low KH they can push pH down markedly, which is why natural blackwaters are both soft and acidic.

Light attenuation

CDOM absorbs most strongly at short wavelengths, from blue into the ultraviolet, which is why humic-rich water looks yellow-brown rather than blue. It reduces the light reaching deeper water, and high concentrations can limit algal and phytoplankton growth, although in lakes moderate CDOM levels are reported to enhance photosynthesis in some cases. CDOM also absorbs UVA and UVB and so shields aquatic organisms from DNA damage. Ultraviolet light breaks CDOM down, gradually bleaching its colour and releasing small molecules and nutrients that microbes and algae can use.

Health claims: what the evidence shows

Antimicrobial and health-promoting effects are often attributed to tannin-stained water. Tannins do inhibit microorganisms, which is one reason they slow the decay of plant tissue. A 2008 review of dissolved humic substances in aquaculture and ornamental fish breeding found that they can improve fish condition, reduce stress-related damage, detoxify heavy metals and help lesions caused by bacteria and parasites heal faster, and that some parasites, mainly fungi, appear to be directly affected. The same authors, however, describe the reports as diverse and sometimes inconsistent, state that the mechanism is unclear, and conclude that humic substances are not real alternatives to conventional therapeutics.

Other studies show that the effects depend on dose and conditions. In eastern rainbowfish (Melanotaenia splendida splendida), humic substances at 10 and 20 mg/L reduced respiratory stress at moderately low pH, but at pH 3.5 in soft water and pH 4 in hard water they increased morbidity. A synthetic humic substance caused slight, dose-dependent feminisation in the swordtail Xiphophorus helleri. For Terminalia catappa leaf extract, an 8-week trial with male Betta splendens found that 0.25 g/L did not affect growth or blood parameters and was associated with more frequent bubble-nest building; an oral toxicity study in red hybrid tilapia found organ tissue changes at 125 mg/kg body weight and above, but not at 31.25–62.5 mg/kg.

Removal with activated carbon

Activated carbon adsorbs dissolved organic compounds on an internal surface that can exceed 3,000 m² per gram, and carbons with a mix of macropores and mesopores are used specifically to adsorb tannins and remove colour. It binds most inorganic ions, strong acids and bases poorly, so it clears colour and organics without restoring lost hardness or alkalinity. The adsorptive capacity is finite: spent carbon must be replaced or regenerated, and industrial thermal regeneration at 800–900 °C burns off 5–15% of the bed each cycle. For aquarium use, Wikipedia advises carbons with low soluble ash content to avoid leaching metals or promoting algae. Tannins also interfere with the molybdate silica test and other colorimetric tests, so stained water can give misleading readings.