Humic substances. Sugars and starches are highly digestible, but not all plant matter breaks down so easily. Humic substances, often called humins or "humic acids," when they are solutes, are a generic catch-all designation for a complex range of dark-colored, variously soluble aromatic organic molecules that result from the incomplete decomposition of plant materials, notably celluloses and lignin. Some polymerization of small molecules also plays a part in composing humins. Humic substances are almost 50% carbon by weight. The source of most of these particulates and humic acids in water is in terrestrial leaf litter through which rainwater percolates; some of this leaf litter gets washed into streams, where it continues its breakdown into humus. Humic substances are only very slowly broken down in water by a community of various bacteria and fungi. The end product will be CO2.
Humic macromolecules are as various as snowflakes: perhaps no two molecules are precisely alike. The two dominant functional groups in these massive molecules are carboxyl groups and phenolic ones. These sections of the complex structures have differing qualities that render the molecules polyelectrolytes; that is to say they are highly polarized, with surfaces that are positive and negative at different sites, though they have an overall negative charge at the pH values of natural freshwaters. Consequently, humic substances are chemically highly reactive: their negative-sum surface electrical charges "pull" ionic heavy metals from the water in the process we call "chelation" and are also sorbed to metallic oxides.
In the aquarium humic substances contribute a desirable or undesirable golden tint to water, which becomes acidic "blackwater" when humins are sufficiently concentrated. They affect the pH, tending to lower it, and have some additional softening effect on water that is not very highly buffered; humins affect the cycling and bioavailability of chemical elements; they repress many bacterial populations and affect the zooplankton: acidic blackwaters rich in humic acids have characteristically low populations of bacteria and a depauperate zooplankton.
Insoluble humins. Not all of the organic substances do dissolve in water. The insoluble residues of plant and some animal structures begin to be called "humus" when they are sufficiently broken down that their original biological sources are no longer recognizable and their nitrogen-containing compounds have been biodegraded. "Humus" is the brown floclike residue of compost, formed by these complex persistent organic compounds, which are resistant to further decay, or comparatively "recalcitrant," which is the ecologist's term that is used in contrast to "bio-degradable." Humic matter composes 70% to 80% of the organic material in topsoils; it's the humus component that makes a good loam so black.
Sources of humic substances. Soluble or not, all these large complex organic molecules were originally formed in plants by the chemical linking of many smaller ones, a process called polymerization. Water-soluble humic substances get leached out of leaf litter by rain. The humic acids in tropical blackwaters originate almost entirely in terrestrial plants and have been flushed from leaf litter into streams. Humic substances in water derive especially from the cellulose in stems and leaves and the lignin that makes wood strong and stable. Part of lignin's structure consists of polyphenolic groups, some of which are retained in humic substances.
Aquarium sources of humic substances. Peat tea, or peat in the filtration, is not the only source of humic substances in the aquarium. Fishkeepers often notice how some woods, such as mopane, release humic stains, pleasing or not. Dry oak leaves can be collected in fall and steeped in boiling water to get a stronger tannic tea than you'll get from peat. Commercially packaged "blackwater extracts" are based on humic substances, though they generally contain other ingredients besides. In my tanks I also get the benefits of humins from leaf litter, Osmunda fiber, coconut shells— and even from used green tea bags in my filters.
Humic substances have multiple negatively-charged sites. Get an idea by checking out a scanning electron microscope image of the surface of a humate— a dehydrated solid humic acid molecule.
Fulvic acid. "Fulvic acids" are sometimes needlessly separated out in aquarium literature. It would take a specialist chemist to correctly divide up humic acids into their several subclasses, such as tannins and fulvic acids— the sub-set of humic acids that are designated "fulvic acids" being even more soluble than humic acids in general. It's merely a functional distinction: to separate humic from fulvic acids, first humic acid has to be extracted from compost, peat etc, using sodium hydroxide, then, when enough acid is added to reduce pH to ~2.0, dissolved organic matter will start to flocculate as humates. What remains in solution is defined as fulvic acid, and this extraction technique is what effectively separates humic from fulvic acids. You see right away that this isn't happening in any aquarium.
Interactions. Humic substances interact with over 50 of the elements in the periodic table. These include plant nutrients, heavy metals and the halogens. All humic substances tint the water yellow to rusty brown, and all are able to chelate positively-charged multivalent ions. They can do this because the negative charged groups present in all humic substances attract cations that have a multiple positive charge, such as iron (Fe++ and Fe+++), calcium (Ca++) and magnesium (Mg++). Cations with a single charge, like sodium (Na+), aren't affected; that figures: you know that a peat filter won't "de-salinate" water. Once these divalent cations are bound to a big humic molecule, they have been taken out of circulation, as it were. If the humic molecule itself were adsorbed to granular activated carbon and then removed from the filter, some permanent water softening would be achieved.
Humic "acid." The "acid" designation of these humic substances is owing to those multiple negatively-charged sites I mentioned. When they attract and bind positively-charged ions (cations), such as iron and manganese, the multiple positive charges of the metallic cations can "knock" the more lightly-charged H+ ions off their sites on the humic molecule. When humic substances take out of circulation some of the calcium (Ca++) and magnesium (Mg++) ions that make water "hard", they do this by release of hydrogen ions in exchange, and the presence of the extra H+ ions in the water gives humic "acids" their acidifying effect. This is also how the humic substances— most familiarly represented by peat— have a softening effect, by trapping calcium and magnesium ions. And that same binding action takes up the multivalent ions of heavy metals that are toxic to fish and plants, including cadmium (Cd++) and lead (Pb++). Humic acids chelate them too, and this action detoxifies them. Humic acids denature proteins: that's the "tanning" effect of tannin. Humic acids have some anti-microbial properties, too, which can make them useful as anti-inflammatories in medicine.
Humates— solids like floc compost or sphagnum peat moss— in a substrate mix will increase the cation exchange capacity (CEC) of the substrate. Humic acid is one of the end products of organic decay because it has great resistance to further decomposition. "That character is a plus in the anoxic lower reaches, where anaerobic decay processes may generate H2S." according to Dr Wayne Kussow's assessment of "Humate and humic acid" archived at the Texas A&M horticulture website.
Chelation. Humic substances also contain N, K, and S groups that can become available to plants. Humic substances chelate micronutrients that would otherwise soon precipitate out of solution and become unavailable to plants. Tannins that bind iron frustrate water treatment engineers like Mike Keller, who can't filter out chelated iron, but should please aquatic gardeners. You might be interested in Keller's article "Water conditioners: tannin removal, trial and error for the best application" originally in Water Conditioning and Purification, November 2002 (p.26).
Besides being good chelating agents, tannins are such excellent dispersants and emulsifiers that they are used as nutrient vehicles in hydoponic plant culture. Humic acids stabilize colloidal clay and keep it bound up in the substrate (which helps keep the water clear). Fulvic acids form complexes with iron and colloidal clay and keep the iron in the water trapped in the substrate.
Humin-intolerant fishes. In case you're getting the impression that humic substances are always desirable, it's good to remember that those fishes that have evolved in alkaline waters or brackish ones are often sensitive to humic substances in the water and don't tolerate them over time: that would include Rift Lake cichlids, many Central American (but not South American) cichlids, and guppies and other livebearers.
Oxidizing humic acids. "Oxidizing agents such as chlorine are effective at chemically breaking down tannins" Mike Keller remarks in the article I've mentioned (p. 31). A fishkeeper's question is whether H2O2 nullifies the good characteristics of humic substances. How much the tannins are oxidized by such agents as hydrogen peroxide and potassium permanganate depends on their concentration and retention time before their action is expended, Keller says. BioAg Technologies' online pamphlet suggests that oxidizing humic substances (using potassium permanganate or hydrogen peroxide) breaks some molecular surface bonds, creating additional negatively-charged sites for nutrient cations to bind to, but that oxidizing also frees colloids and silicates from their humic acid bonds.
If you were intent on removing tannins from your water source, you could depend on reverse osmosis for a while. Tannins are high-molecular weight molecules, thus are rejected by reverse osmosis membranes. But tannins will quickly foul R/O membranes. If you were intent on removing tannins from aquarium water, you'd use granular activated carbon, though there are also some specialized acrylic resins in the chloride form that work through a combination of ion-exchange capacity and adsorption and are regenerated at frequent intervals in salt brine.
Links. The International Humic Substances Society offers a brief overview "What are humic substances?" BioAg Technologies' online pamphlet on humic acid structure and properties explains the soluble humic acid product, BioLiquid Complex, that they are marketing to farmers. It is derived from leonardite, a mineral akin to lignite, which is already naturally partly oxidized, then further treated with potassium hydroxide and hydrogen peroxide. The company claims that leonardite is more active as a binder and carrier than humic acid derived from peat. For fishkeeping purposes, the product might be compared to a blackwater extract.