Dissolved organic substances
Besides dissolved gases and dissolved minerals, the third component dissolved in all waters, both natural and captive ones, comprises various forms of dissolved organic carbon (DOC). DOC is composed of all the soluble carbon-based molecules that have been assembled by living organisms.
More than just a nutrient, carbon is the basic building block of all life. The biological cycling of nitrogen, phosphorus, etc are sketched for you in the "Nutrient Cycles" pages: carbon is also recycled. The vehicle for carbon cycling is dissolved organic carbon (DOC). Some organic carbon molecules in the aquarium water are quite transient, such as the biodegradable amino acids, polypeptides and proteins, sugars and carbohydrates. Other forms of DOC are the more resistant and permanent humic substances. DOC may originate on land, in leaf litter and other organic debris, and then get washed into the water. Or it may originate in the water, in algae and plants, or in bacteria or planktonic animals.
In its dissolved organic forms, carbon is constantly being decomposed, cycling down until it is finally completely oxidized to carbon dioxide and molecules of water. Carbon dioxide and carbonate minerals form the carbon cycle's inorganic phase. (Similarly, the phosphate cycle has its own inorganic phase, orthophosphate, and the nitrogen cycle has an inorganic phase in water, ammonia/ammonium.) Once carbon has been oxidized to CO2, it is ready to be taken up by plants and algae through photosynthesis, to be built into organic carbon once more. In the broadest, simplest sense, that's the carbon cycle in fresh water.
So, DOC can be separated into two groups: the humic substances and every other kind of dissolved organic carbon, the transient kinds.
Non-Humic Dissolved Organic Carbon. Not all organic carbon is built into the large molecules you might think of when you think of "organic" molecules: simple amino acids are also part of the DOC loading in water. Other examples of non-humic DOC are substances like carbohydrates (simple sugars and starches), proteins and the peptides they are made from, fats and waxes (lipids), resins, pigments, and other carbon-based substances of low molecular weight. In general the non-humic organic substances are more easily assimilated by micro-organisms. Down at the base of the trophic web, these forms of DOC constitute a fundamental food source for bacteria and fungi, and so they exhibit rapid flux rates. In other words, they don't last long in the aquarium before they are assimilated by some organism. So, you don't need to worry about these organics building up in the planted aquarium. Of the various forms that dissolved organic carbon may take, the humic substances are the only ones likely to build up as metabolic end products in a dynamically balanced "natural" aquarium.
Humins in DOC. The humic substances constitute the bulk of organic matter in natural and captive freshwater systems, partly because they don't break down so easily. Humic substances may account for as much as 95% of the total dissolved matter in blackwater systems, where the water is so soft, so free of electrolytes, that the concentration of humins is greater than all the inorganic ions in the water put together. Humins are described as acidic, dark colored, partially aromatic, chemically complex substances with high molecular weights, an intentionally vague and inclusive definition.
Humic DOC participates in many aspects of water chemistry. It is often underestimated as a source of acids, in the acid-base buffering of freshwaters, particularly in soft water. DOC chelates trace metals, binding them in water-soluble forms. Its golden to brown color may even affect the transmission of light. All the pages in the rest of this folder concern humic substances.