There is one animal that has only 1 eye which is the Copepod.
Copepods are a group of small crustaceans that are found in nearly every freshwater and saltwater habitat.
Some species of Copepods are planktonic, some are benthic, a number of species have parasitic phases.
Copepods are tiny, white flea-like critters that can be found skipping around in the aquarium.
They are detritivores, so it is beneficial to have them in the aquarium as part of the tank's ecosystem.
Microcrustaceans (or “pods”) in general, and copepods in particular, are an integral component of aquatic food webs.
This is so in virtually every marine environment including coral reef habitats.
Therefore, pods in your reef are a healthful addition to any reef aquarium.
A variety of different freshwater and marine fishes are susceptible to infesta- tions from these copepods. attachment and/or feeding by the cope- pods is common resulting in frayed fins, gill hyperplasia, and patchy epidermal damage and necrosis.
Infections with secondary pathogens often occur.
Copepods further serve as intermediate hosts for several species of cestodes of fish, including the fish tapeworm Diphyllobothrium latum, which also infects humans.
However, human infection does not occur through infected copepods, but by the consumption of raw fish.
If enough Copepods and Rotifers are added to your aquarium and fed regular doses of Phytoplankton, they will reproduce and create a sustainable live food source for your Fish, Corals, and other Inverts.