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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

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
*AAP Paracide
*AAP Dyacide (Dylox)
*AAP Piperazine/Pipzine

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

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

Nutramar Nori- Seaweed

An excellent food source for many Herbivores

Aquarium Lighting, Complete Information

Aquarium Power Head PumpAAP JT-132 Pro Pump; 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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Dropsy in Fish; Swollen Betta, Kidney Infection, Osmoregulation

Betta, Platy, goldfish, fish with DropsyDROPSY in fish; prevention, causes, and possible treatment of this malady that is often a symptom of other problems.

By Carl Strohmeyer
Updated 11/24/21


I've been professionally treating many fish diseases, conditions & syndromes since 1978.
Dropsy is a condition I've treated for clients many times over the decades and discussed with other leading professionals at seminars too often at seminars and industry trade shows.

What is Dropsy?

Dropsy is basically a "Condition" or "Syndrome".
In other words a symptom with many causes, often a mix of causes such as diet, osmoregulation, bacterial pathogens such as Aeromonas and generally affecting the kidneys.
As noted, most often Dropsy is Kidney related, which results in swelling and fluid retention due to poor kidney function results in the classic "pinecone" look of fish sick with Dropsy.
Poor osmoregulation is usually the second most common cause, followed by diet, and in more rare instances digestive, and maybe liver malfunction/infections.
Often it is a combination of infection of the kidney, poor osmoregulation (including oxidative stress), and poor diet (which often is the result of too high energy levels and poor protein optimization).

The loss of ability for osmo-regulation of electrolytes is often a contributing cause, which is another reason for correct positive mineral ion and trace element levels.
Please see these two articles:
*Aquarium Chemistry; Calcium, Electrolytes, GH, KH, & more
*Do Fish Drink; Proper Osmotic Function .

Sadly a person who may or may not have some expertise is posting on YouTube video comments (including AAPs channel) touting his experience with API (a known discounter of limited aquarium medications, certainly not a seller of higher end pharmaceutical grade products such as Aquatronics or SeaChem).
He attacked my experience and expertise on this subject calling it "True Dropsy" when in fact the industry has NEVER considered Dropsy a "True" disease (which is why I doubt his so called expertise and believe he is more likely an internet troll that are so common of late trying to make a name for himself posting on many YouTube channels dealing with this subject).
He also stated that True Dropsy is not curable. Well, since Dropsy is actually a condition and not a true disease per say (example would be Septicemia in fish which is also a condition with many possible causes), some causes of Dropsy can and as a MATTER OF FACT have been cured when caught in time. Unfortunately some causes are indeed incurable no matter what one does for their fish, and even if caught early on.
This is also why prevention (as you will read later in this article) is key, as I had very few cases of Dropsy of ANY cause when certain procedures were followed.

Generally due to the cause or area of infection (or organ failure), Dropsy can often be very difficult to treat, especially if caught in an advanced case in the fish (usually in advanced stages euthanasia is your best choice).
What the aquarist often observes is a “pinecone” swelling generally caused by fluid building inside the body cavity (often involving the Kidneys), for this reason, reducing this swelling is an important step in effecting a cure.
I have heard of Minocycline also being recommended for this (although it can be effective). I do not recommend this as Minocycline has been shown to cause serious damage to the kidneys, which is the last thing you want to do to a fish suffering from Dropsy or even suspected of this malady.

Information on Minocycline:
Use of Minocycline, Maracyn 2 for fish treatment

I would also note that since Aeromonas bacteria is a common cause of infections (along with the parasite Mitraspora cyprini, more commonly found in pond fish) that result in Dropsy and since this bacterium is often anaerobic; maintaining good circulation, aeration and overall good tank hygiene goes a long ways in treatment and even further for prevention (since Dropsy is difficult to treat and cure).
Please read more about optimum tank conditions in the prevention section further into this article, as I have been able to prevent Dropsy much more successfully (as per controlled tests) than actually treat a full blown case of Dropsy!.

Sometimes liver issues are blamed for Dropsy, which is certainly possible, however liver malfunction and infections generally do not result in the classic "pinecone" Dropsy appearance, rather the fish may bloat slightly and almost always results in loss of color, loss of appetite, and the fish tends to hide more.
One of the functions of the liver is to remove toxins and this is why the symptoms I describe are more prevalent with liver issues.
Generally if liver problems are the cause, successful treatment is rare

Other issues blamed for Dropsy include swelling for egg impaction, swim bladder issues, Mycobacterium Tuberculosis, & Costia.
However, while the later two in particular can be similar, based on my decades of practical experience dealing with Classic Dropsy these still do not produce the classic symptoms and more importantly treatment is quite different.

This said, besides treatment for a possible underlying infection, you want to take steps to remove the swelling.


Here are the steps I would take to have chance at defeating the causes of Dropsy in less advanced stages.
ALL STEPS ARE IMPORTANT, Skipping even a couple lowers effectiveness considerably.
These steps apply to ALL fish, not just Bettas
Success rate from my experience when ALL steps are followed (which is in the 100s of cases) is about 50%. But I must qualify this by stating that in cases where the fish is not eating and non responsive is closer to ZERO while in cases where the fish is responsive, eating and otherwise doing relatively well is much higher than 50%,


The Aeromonas bacteria (that is often present in healthy aquariums) can cause infections that will manifest this way in poor water conditions, especially in aquariums with poor circulation and high amounts of DOC (dissolved organic compounds) along with a high "Bio-Load" as Aeromonas Bacteria can be anaerobic and thrive in low oxygen, high dissolved organics conditions.

Aeromonas Infections in Fish

Please also reference: Bio load in Aquariums

So maintaining a healthy aquarium with regular water changes, good filtration (if possible, as this is why I see more cases of Dropsy in Betta kept in a bowl), and maintaining optimum water parameters; Ammonia and nitrites 0, kH 50+ ppm, GH 100+ ppm, & nitrates under 40 ppm (the lower the better, so 20 ppm or less is better yet).

A water parameter that is often missed (as the more obvious ammonia and nitrites are usually noticed) is proper calcium and electrolytes (positive mineral cations). These mineral Cations help balance oxidative stress which can have an impact on Kidney function & fluid retention. This can be tested in rH/Relative Hydrogen Score with healthy aquariums should have a rH reading between 23 & 26

If RO or RO/DI is used (or drinking water that is nothing more than RO water with a few minerals added for “taste”), there are usually insufficient electrolytes and calcium for proper osmotic function and fluid retention can result, which will then lead to kidney infections. Make sure to properly re-mineralize if RO water is used

Please read these articles for more about this aspect of fish care:
*Proper Osmotic Function; Use of RO Water
*Aquarium Redox & rH
* AQUARIUM CHEMISTRY; The importance of Calcium, GH, KH, Magnesium to Fish

All this goes a long way in prevention of Dropsy and other diseases, especially when poor osmoregulation is the direct cause or even indirect cause of Dropsy.
By indirect I mean opportunistic infections getting a foothold internally in your fish due to poor levels mineral cations & buffers present in your aquarium water.

Use of true level one UV Sterilization..
Similar in benefits to mineral Cations, but not having a direct impact on osmoregulation, the use of level one UVC Sterilization lowers oxidative stress in the water column, which over time has a direct impact on fish health and longevity, including likelihood of issues that can result in Dropsy symptoms.
*Aquarium/Pond UV Sterilization

A proper diet makes a large difference here.
Do not feed your fish "meat based" proteins. I recommend aquatic based proteins such as whole menhaden or white fish meal, shrimp or even the proteins found in spirulina algae.

As a basic Betta diet I recommend Sanyu or Hikari Betta Gold pelleted foods.
There are many other quality Betta foods as well, although I do not recommend flake foods for bettas as their staple diet.
However Flake Food such as Spirulina based Spirulina 20 are excellent when used as a "fish food slurry/soak" for frozen or freeze dried brine shrimp, worms or other carnivorous diet food that Bettas prefer

For other fish, fish food guru Clay Neighbor's AAP Custom Premium & Spirulina 20 are excellent staple diets (in fact, easily the best when the science of fish nutrition is applied including energy levels and fiber)).

Poor quality proteins (or better; unusable amino acids for fish) can lead to digestive problems or Renal failure, which CAN lead to the symptoms of Dropsy.
All proteins are made up of amino acids, some are usable (by fish), and some are not.
Those that are not are disposed of by kidneys in the fish. This can lead to renal failure or infection.
This is where highly digestible foods come in and more importantly fish food that optimize/limit their ingredients such as Clay Neighbor's Custom fish foods.

Unfortunately even among so-called premium fish foods, the common method is to utilize "the more the better" method of ingredients, in particular proteins. This can and DOES slowly result in kidney damage that can result in many opportunistic infections, including those that result in fish dropsy.
Oxidative stress to the kidneys over long periods of time by fish foods too high in energy levels is something to consider when looking to prevent Dropsy.
While changing to a better diet once a fish is diagnosed with dropsy is too little too late from my experience, changing to a better more optimized diet is certainly good prevention for dropsy and other fish diseases for your existing fish.

One of the few if only fish foods that truly optimize/limits ingredients is Clay Neighbor's "AAP Custom All Natural Optimized Fish Food" (there are a few copycats that do not know the exact formula that are not quite up to par here).

Also it is often a good idea to soak dry foods in water for 5 minutes prior to feeding as this will remove air that can lead to infections of the digestive tract (goldfish in particular).

For more information about proper nutrition, see this article:
"Fish Nutrition; What ingredients are needed for proper fish growth and health".
See also this section:
Fish Nutrition; FD/Frozen Foods Spirulina Slurry

Other suggested reading:
Spirulina Algae; The Aquatic Health Benefits for Tropical, Marine and Goldfish.

Fish Food Recommendations, product links:
*Clay Neighbor's "AAP Custom All Natural Optimized Fish Food" (includes Carnivore for Bettas)
*Spirulina 20 Fish Flake Food
*Hikari Betta Bio-Gold
*Hikari Spirulina Enhanced Brine Shrimp

The above is a VERY short list of quality, highly digestible fish food diets, so please read the article I suggested earlier about Fish Nutrition!!

To summarize prevention; I have seen very few cases of Dropsy in the literally 1000s of aquariums I have maintained in regular contracts over the years. This is because I followed ALL Steps outlined in the AAP Aquarium Disease Prevention Article, and the facts of this article speak for them self, especially when it comes to Dropsy.
Reference: Aquarium Disease Prevention

HOWEVER, I have seen MANY cases of "Classic" Dropsy (as well as Dropsy like problems) when non regular service customers call me out to see why their fish are sick and I often will observe very poor water conditions feeding practices, etc. (basically NOT following the steps in the article outlined above).
The reason is simple for my regular contract customers success, I have always maintained my tanks with regular cleanings, proper electrolyte levels, a balanced Redox, proper fish nutrition, and often UV Sterilization.

Other Recommended Reference & Product Sites

AAP AAP Nitrofuracin Green, Synergistic Nitrofurazone, Sulfathiazole Sodium, & Methylene Blue Full Spectrum Treatment
Our special formulation of Nitrofurazone, Sulfathiazole Sodium, Methylene Blue and sodium chloride.
Wide spectrum. Anti-microbial, anti-protozoan, anti-bacterial, anti-fungal. Good for newly arrived fish in quarantine situations.
Also good for healing wounds and ammonia burn on newly arriving fish. Works well for sores on fish in Koi ponds..

AAP AAP Nitrofuracin Green AAP Nitrofuracin Green; Synergistic Nitrofurazone, Sulfathiazole Sodium, & Methylene Blue Full Spectrum Treatment

* “Aquarium Disease Prevention”
In controlled test/studies, the incidence of Dropsy was almost non-existent where all points outlined in the above Disease Prevention article were followed!!

*Aquarium Redox Potential

*UV Sterilization, Sterilizer Use; The importance in fish disease prevention

*Freshwater Aquarium Care Basics & Information

*Aquarium Filtration Information

Aquarium Answers, Fish anatomy, fin identificationFish Anatomy For further help in understanding the anatomy of fish (so as to know where the Kidneys are located), please read this article.

*Planaria & Detritus Worms in Aquarium

The best in Quartz, Under gravel, and Titanium Submersible Heaters:

Aquarium Heaters

TMC V2 RO Filter systems; the very best you can buy with TDS meter:

Reverse Osmosis Aquarium Water Filter, TDSReverse Osmosis Aquarium Water Filters; with TDS Meter

AAP AquaRay Ultra Premium Aquarium LED Lights
Highest in PUR, The ONLY LED with an IP67 rating or higher for water proofing along with a full 5 year warranty to back them up!
Why purchase brands without this rating such as the Finnex, Current, or Fluval only to be essentially placing an electronic light emitting device over your humid aquarium with little or no guarantee? In the long term, you WILL PAY MORE!


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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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Aquarium Chemistry; In Depth Information

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Free Shipping AAP

*Sponge Filters that far outperform all other brands or DIY:
*Patented Lustar Hydro Sponge Aquarium Filters

UV Replacement Bulbs/Lamps Directory:
*UV Bulbs directory

*A CLEAR POND; Care & Information

Fish Food Information:

Aquarium & Pond UV Sterilizer Use Articles
-Unique articles such as unique UV ideas as well as dispelling myths

Directions/ Instructions/Downloads; Aquarium Pond Products

Aqua UV versus TMC UV Sterilizer
VIDEO: Aqua Pond UV vs TMC AAP Pond UV Clarifier Sterilizer

A comparison of the two top large pond/aquarium-system UV Sterilizers and why the AAP/TMC comes out as the best when price and dwell time is considered

Best Aquarium & Pond Medications
VIDEO: AAP Professional Aquarium & Pond Medications


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