Colloids.

Colloids are silty particulates small enough to remain in suspension indefinitely, too small to settle by gravity alone, though they could be separated in a centrifuge. Colloidal particles are partly kept in suspension by their electrostatic repulsion: they bear negative charges.

Colloids are powerful light-scatterers that contribute to the initial cloudy water of a newly-set up tank. Colloids are distinct from microscopic grains. They are platelike, though it takes an electron-scanning microscope to see how they stack together like paving flags at a stone yard. With their large surface area per unit of mass, their geometry tells you that, ounce for ounce, colloidal particles have much greater surface area than sand grains.

Finely-dispersed colloidal particles make excellent adsorbents, binding various molecules or ions onto their surfaces. In other words, colloids have a high cation exchange capacity (CEC). They are used in wastewater management to remove some soluble contaminants from water. In the aquarium, you want the colloids in the substrate. In the substrate, colloids will bind phosphate and some other nutrients for plants in the rootzone. you'll find more about this role of phosphates in the "Substrate" folder.

Colloids also provide a mineral substrate for biofilm to grow upon.