What you are most likely seeing are Copepods and/or Amphipods. These are shrimp-like crustaceans that dwell in the substrate as adults, but during their larval and juvenile stages are most often free swimming.
Because of the complex number of families, genera, and thousands of species contained within the Phylum Arthropoda > Class Crustacea > Subclass Copepoda, and Subclass Malacostraca > Order Amphipoda / Order Amphipoda groups , we are not going into detail about the taxonomy of these organisms, but here are some basic facts about these tiny crustaceans.
There are both pelagic (free swimming), and benthic (bottom dwelling) bugs.
Copepods occur in all types of aquatic ecosystems; freshwater, estuarine (brackish) and marine.
Amphipods are mostly found in marine ecosystems, but there are some freshwater and terrestrial species.
They are just a few of the tiny animal organisms that make up zooplankton, which contributes to the make up of plankton.
These creatures eat phytoplankton (tiny plants and algae that also help make up plankton), small microzooplankton (the division of zooplankton that are smaller than 200 microns, or 1/127th of an inch in size), and detritus.
Only a few of the thousands of species of copepods and amphipods known are carnivorous or parasitic, and these are rarely found in a saltwater aquarium system.
For many saltwater fish and other marine species, copepods and amphipods are a primary food source, both in nature and in captivity.
Because these tiny organisms are a natural part of the plankton food chain in the ocean realm, they are naturally going to occur in a saltwater aquarium environment. They are also micro-cultured as food for various species of adult marine animals, as well as used and tested as a food source in the research of culturing and rearing all kinds of tank-raised fry.
Copepods and amphipods most often appear in closed aquarium systems after live sand and/or rock has been added.
