Nutrient cycles. The pages in the "Energy Flow: food web" folder that follows this one will outline some of the roles that living organisms play in transforming energy as it passes through the food web and is finally dissipated. But first, in the material within this folder you could think of the paths energy takes simply as a series of chemical transformations. Employing the energy that comes into the aquarium in the forms of light and heat, and the potential energy represented by the nutrients that you directly introduce, the chemicals essential to life are transformed through the cycles of nutrients, especially of carbon and nitrogen, but also of sulfate/sulfide and phosphates. Iron is highly reactive, too, ready to shift from phase to phase.
Energy is eventually lost, irretrievably dissipated in the process called entropy, but the nutrients are conserved by being transformed in the nutrient cycles. The aquarium is a closed system, unlike the open system of natural waters in this respect; nearly everything that goes in, remains there, no matter how it may be transformed, until it is exported by the fishkeeper, either by pruning the plants, siphoning away debris or doing a partial water change.
Carbon cycle. Carbon is the stuff of life, for all us carbon-based lifeforms, but I haven't discussed the carbon cycle on its own. Like phosphate and sulfur, carbon has an inorganic and an organic form. Its inorganic phase is in the carbon dioxide/carbonate buffering system, represented in the form of dissolved CO2 and "alkalinity". In its organic phases, carbon cycles through all the lifeforms represented in the "Energy Flow" folder that follows this one in the menu at left.