Decomposers, the other half of the food web

Decomposers form the other half of the food web, as the circuit swings downward from the complicated integrated systems of plants and animals back towards molecular simplicity.  The familiar trophic web, characterized by photosynthesizing producers that support a network of energy consumers, and building up to the top predators, the fishes, is complemented by another trophic web, in which organic material is decomposed.
 
Before plants and other photosynthesizers can re-use them, the organic tissues of all living organisms have to be disassembled, broken down into molecular fragments, stage by stage. At the bottom of the energy cycle the molecules are small and simple enough to be assimilated by plants and can begin their upward journey once again. Though ordinarily you net out the corpse of a fish and export all the potential nutrients it represents, there is a perpetual rain of minute and microscopic corpses, of uneaten defrosted brine shrimp, shreds of plant stem and leaf, fish feces and the bacteria it contains. Uneaten flake feed makes its contribution too. So, to start at the top, this second half of the energy cycle begins with the scavengers and detritivores, including snails and most flatworms. Fishes aren't left out of this. They are opportunists that will devour any fresh corpse. Top-level scavengers and opportunists break up the detritus into smaller particles, which multiplies the edge areas available to microbes. Scavengers metabolise their share, and eventually their undigested fecal excretions are taken up by microbial decomposers.
 
Detritus feeders: "The Diet of Worms." Detritus ("mulm") describes all the once-living particles of organic matter, however small, which have lost their former structure but are not dissolved. (Ecologists calculating the energy budget of  ecosystems will extend the concept of detritus to include dissolved organic carbon [DOC]). Detritus is in the process of decomposition to humus. In a deep lake, detritus may largely consist of algae and zooplankton that have drifted below the sunlit zone. In sunlit but shallow waters, plants may be in control, and the detritus will largely consist of decaying plant tissues. In a rainforest stream, by contrast, most of the detritus will be terrestrial leaf litter and clay particles washed into the watercourse. Detritus also includes shreds of animal tissue: insect wings, carcasses of planktonic creatures. The detritivores who make a living from this broad resource are also getting nutrition from its easily-digestible associated bacteria, which offer concentrated sources of essential vitamins, amino acids and polysaccharides in bacterial coatings.
 
Detritus gatherers are essential players in the food web. In spite of our aquarium focus on the specialized "algae-eaters," freshwater habitats generally support few large-scale grazers. Ducks don't make a dent in the duckweed, nor do your barbs. As much as 90% of plant production would die and go directly to the detritus feeders, except that you short-circuit the system by removing leaves as they yellow, exporting the nutrients that they represent.
 
In natural systems there are several techniques grazers use for handling such a high-bulk resource of fairly low nutrient value. Suspension- or filter-feeders intercept fine sediments even before they have settled out. Microscopic rotifers and tiny daphnids operate this way, but so do massive Singapore shrimp, Atyopsis moluccensis. Once the detritus is part of the substrate, two other techniques come into play. Shredders selectively process larger particles of plant debris. By contrast, collector-gatherers, such as oligochaete worms, take it in bulk, gobbling their own weight in detritus daily, more or less unselectively. The collector-gatherers pass the finer particles quickly through their systems. Many detritivores are equipped to switch from one technique to another, as opportunities arise.
 
Algae pass through bloom-and-bust cycles, so do diatoms, and tropical waters are often quite seasonal. The concept that "temporal and spatial differences in the availibility of food sources for detritivores are main regulating factors for communities of benthic invertebrates" was the main idea in a thesis of J. H. Vos, "Feeding of detritivores in freshwater sediments". If you read Vos' introduction and concluding remarks, you'll see how much I owe to him here. Vos' introduction stresses how important the microbes are at every stage in the decomposition of organic material, both by converting into biomass otherwise unavailable materials like cellulose and by assimilating soluble components that would otherwise disperse and be lost to organisms through chemical oxidation. But some organic substances resist complete disassembling. They are transformed by bacterial action into humic substances, which accumulate in the water and also help create humus in the substrate. Humic substances dissolved in the water account for about half of the dissolved organic carbon. (The remainder is accounted for by transitory simple compounds in solution: carbohydrates, amino acids, peptides, carboxylic acid, hydrocarbons, and the like.)
 
All these organisms that break down organic material are operating by aerobic metabolisms. In fact they require so much oxygen that when water is rich in organic substances that are in the process of being broken down, it may become temporarily depleted of oxygen. This can lead to mass fish deaths in polluted rivers, but hopefully will not occur in your aquarium. Each level of decomposition is somewhat more efficient in energy transfer than levels in the consumer web are, but it does also contribute its share to entropy. If this weren't true, to take just one example, our aquariums would build up high levels of nitrate even faster than they do.