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Cyclops, Are these freshwater copepods dangerous in an Aquarium?

Information about Cyclops and also Predatory Damselfly larvae that occasionally find their way into aquariums and especially ponds

By Carl Strohmeyer

Updated 8-26-14

QUESTION: I have these bugs in my aquarium; they are very small about 2-3 mm long with one black eye in the middle of its head. It is grayish in color. It goes jerking through water in rather spastic motions.
What are these?

Is this your bug?

Cyclops Copepods in Aquarium

This is one possibility, which is a Cyclops (a type of copepod); they range in size from 1-3 mm. It is called a Cyclops because of the single eye spot. This female carries two packages of eggs near the base of her tail. The Cyclops has 5 pair of legs and a divided tail-like appendage called a furca (A forked process as the last abdominal segment of certain crustaceans).
Cyclops are crustaceans and related to lobsters, crabs and shrimp.

A little about Cyclops;

Cyclops live in freshwater, such as ponds and lakes. They prefer areas of still water with a lot of algae.


Cyclops are omnivores; they eat algae, small animals, detritus but not usually fish. But weak fry MAY be harmed by these copepods.
Some species of Cyclops are not free-living, existing instead as fish parasites, but these are not common.
Tropical and native fish enthusiasts utilize the cyclopoids as a high protein live fish food, especially for immature fishes, and in a fair twist of fate, some Cyclops species have been found feasting on larval fishes, particularly in fish hatcheries.

For more about proper fish food nutrition:
Fish Nutrition; What ingredients are needed for best fish nutrition, growth and health

Cyclops are generally eaten by Phantom Midge Larvae and water mites.



The female Cyclops carries her eggs at the back of her body. There are fewer males than females present in the population, and the males are smaller.
Females are often observed while swimming to have paired egg sacs, and most of the species are capable of explosive outbreaks when the water temperature and other conditions are optimal.


Damselfly Larvae Another possibility is a larval damselfly, which are much larger, about 1/4" or more (although this does not fit the above description, however I have seen these in ponds).
The larval damselfly is more predatory, but will not reproduce in your aquarium (unless you have damselflies flying around the inside of your home!).
These are best hand removed (or vacuumed). Once these are gone, they are gone!

CONCLUSION (& Removal):

Cyclops can also be intermediary hosts to the Guinea worm (which affects humans, but not generally fish) and fish tapeworm.
These are rare occurrences for the Cyclops, and many aquarists consider them valuable as a fish food and even sell them.
I would not be overly concerned with them for most aquariums as long as good aquarium cleaning methods, such as vacuuming are practiced; in fact as noted earlier they actually make a nutritious fish food.

The exception would probably be with fish breeding. In this case, vacuuming gravel and rinsing off live plants in a mild bleach solution (live plants can handle a 25/1 solution provided it is rinsed off) would be in order. Keep in mind that these copepods do not like areas of high current and do like a lot of algae, so changing these conditions would go a long way in controlling Cyclops.
Gravel Vacuum Product Link: Aquarium Vacuums

If normal maintenance methods fail to remove the Cyclops or similar Copepods, chemical removal methods can be used.

There are four methods I have used and can recommend based on results and safety (there may be other methods):

Do not use Ich treatments or other single cell parasite treatments.

Treatment Method Product Links:
*Cupramine from AAP
*Tetra/Jungle Parasite Guard

For other Fish Food Product Resources:
*Fish Foods; Hikari, HBH, Spirulina 20, Aqua Master, Freeze Dried

Other Recommended Reference & Product Sites

Aquarium Information -A useful source for current Aquarium & Pond Information and Resources

*Aquarium, Fish Parasites, Worms; Planaria, Nematodes, Detritus, Anchor

*Aquarium Ich, Treatment, Prevention

*Planaria or Detritus Worms in Aquarium?

UV-C Replacement Bulbs Page 1

For TRUE High Output, Hot Cathode, Low Pressure UVC Germicidal Bulbs, for aquarium or pond

SeaWeed Salad

An excellent food source for many Herbivores

Aquarium Lighting, Complete Information

Aquarium Power Head PumpSunSun JP-023 Aquarium Power Head Water Pumps

Superior performance and value when compared to many more well known brands such as Hagen or Marineland


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In Chronological order of writing with the newest at the top

  1. How to Treat Sick Fish
  2. Whirling Disease in Fish
  3. Reef Aquarium Chemistry Maintenance
  4. Use of RO, DI, Softwater in Aquariums
  5. Lighting Theory of a Planted Aquarium- RQE, PFY, PAS, & PUR
  6. Aquarium or Pond Bio Load
  7. Tuberculosis in Fish
  8. PUR vs PAR in Aquarium Lighting
  9. Head Pressure in Aquarium and Pond Water Pumps
  10. Fin/Tail Rot For Betta & ALL Fish
  11. Angelfish Virus/Aids
  12. Activated Carbon
  13. Fish Baths/Dips as an aid to treatment
  14. Streptococcus gram positive bacterium in aquariums, Eye Infections
  15. Hydrogen Sulfide
    production in anaerobic De-Nitrification for Aquarium/Ponds
  16. Fish Shipping
  17. Aquarium Size, Fish Stunting
  18. Aquarium Algae,
    BBA & Brown Algae in particular
  19. Aquarium Salt (Sodium chloride) in Freshwater Aquariums
  20. Betta Habitat; Wild Bettas to Domestic Betta environment parameters
  21. HITH; Hole in the Head Disease
  22. Aquarium Protein Skimmers, Ozonizers
  23. Power Head/ Water Pump Review
  24. Molly Disease/ Mollies in an Aquarium
  25. Basic Fish Anatomy, Fin Identification
  26. Aquarium Moving/ Power Failures
  27. Octopus as Aquarium Pets
  28. Aquarium Nitrates
  29. Ichthyophonus protists, fungus in fish
  30. Aquarium and Pond Filter Media
    Types; Mechanical, Bio, Chemical
  31. Aquarium Water Conditioners (also Pond)
  32. Fish Parasites; Trematodes & Monogeneans; Annelids and Nematodes;
    Flukes, internal worms, Detritus Worms (often confused with Planaria), Micro Worms
  33. Aquarium Silicone Application;
    DIY Aquarium Repair & Glass thickness
  34. Pond Veggie Filters; DIY Bog Filter
  35. The difference between Plaster of Paris and Aquarium Products such a Wonder Shells:
    Identification, prevention & Treatment
  37. AQUARIUM TEST KITS; Use & Importance
  38. SEXING FISH; Basics
  39. Chocolate Chip, Knobby and Fromia Starfish
  40. Freshwater Velvet & Costia
  41. Usnic Acid as a Fish Remedy
  42. Aquarium Heaters; Types, information
  43. The Lateral Line in Fish, Lateral Line Disease
    or Head and Lateral Line Erosion (HLLE)
  44. Tap Water use in Aquarium; Chloramines, Chlorine
  45. Can Black Ghost Knife fish give an electric shock?
  46. Bio Wheel Review; Do Bio-Wheels really work?
  47. How do Fish Drink?
    Use of RO Water
  48. Cyclops, and Predatory Damselfly larvae
  49. Betta with Dropsy;
    Treatment and Prevention of DROPSY in all fish
  50. pH and KH problems in African Cichlid Aquarium
  51. Aquarium Gravel, which size?
  52. Blue green algae, Cyanobacteria in Ponds/Aquariums

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