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SEXING FISH, Stimulating Fish Breeding Behavior


SEXING FISH; The differences between males and females (if any).
By Carl Strohmeyer
Updated 9/29/13

I will first point out that the ability to relatively easily sex fish depends on the fish; and for many newer fish keepers sexing can be very difficult.

I would also like to point out that although some articles here at Aquarium Answers are intended to be more in depth, THIS ARTICLE is intended to just cover the BASICS and advanced aquarists will need to seek further information elsewhere.
I provide several links to other sites and articles for more expanded information, please check them out as well

Although information about the breeding of fish is not the main intention of this article, I will provide a few basic factors to consider for many fish developing into a breeding condition (where by "sexing" of the fish is more likely to be successful too).

Important Fish Breeding "Stimulants";

Important Fish Breeding "Water Condition Basics";

Simply put, maintaining good water conditions is a must, this includes 0 ammonia or nitrites, low nitrates, a stable pH, at least some positive mineral electrolytes (some fish need more than others for positive breeding conditioning).

Often successful breeding is performed in bare, no substrate aquarium; this in part aids in the ability to vacuum and wastes as soon as it falls and limits the possibility of devastating Saprolegnia outbreaks in fish or eggs.

Another key point is while many breeding fish prefer at least some quiet calm areas to breed or rest, at least SOME filtration MUST be present. I base this on the FACT most breeders as well as my breeding of fish have been in tanks with some sort of filtration. Generally speaking, an air driven Sponge Filter is your best choice for this.

A Resource for:
Aquarium Sponge Filters

See Also:
*Aquarium Nitrogen Cycle
*Aquarium Chemistry; The Importance of Maintaining Positive Calcium Ion levels
*Freshwater Aquarium Care



Male and female Platy, pineapple sword

In Livebearers such as platys the male has a short tube called gonopodium in place of the anal fin, while the female has a normal triangular anal fin.

Guppies more transparent bodies (although many other light colored livebearers are similar here) are easier than many livebearers to see fry development inside the female.
Gravid female guppy, giving birth Signs that your guppy or other livebearer is about to give birth include a large belly, a box like look from the front view, and a darker gravid spot which is under the pregnant females tail, which many identify as a stained glass window where the fry come out.
As the babies near birth you can sometimes see eyes of the unborn fry as tiny black dots.
Click picture of guppy to enlarge

The gestation period in livebearers, which the interval between fertilization and live birth can vary, however as a generalization this time period is about 4-6 weeks at 78 F. The size of the brood can also vary greatly.
Most livebearers prefer water higher in mineral ion content and although salt is not required by most livebearers, it can be helpful as well.
Keep optimum conditions including proper mineralization as well as supplementing their diets with frozen/live/FD foods along with quality flake & conditioning foods (I would strongly suggest foods high in spirulina such as Spirulina 20 as well as conditioning diets such as AquaMaster Enhance Immunity Tropical Fish Food).

More information about salt use:
Aquarium Answers, Salt in Freshwater Aquariums

As for the fry, I have achieved phenomenal success by simply powering Spirulina Flakes at the time of most of my livebearer breeding, of which I had several tanks of Platties alone. I was using Green Flake Plus which is currently not available, but similar products such as Spirulina 20 still are. I also would supplement them with frozen baby brine shrimp or microworms.

Aqua Master Conditioning Mini Tropical Fish Food-Enhance ImmunityFish food Resources:
*Spirulina 20 Fish Food Flake
*Freeze Dried Fish foods; Worms, Brine Shrimp
*Aqua Master Conditioning Mini Tropical Fish Food-Enhance Immunity

Most livebearers of both sexes will eat their young so separation of all other fish is best and the use of a heavily planted aquarium (live or fake) or breeder trap for the female is advised.
Both swords and platties are capable of cross breeding as are Mollies and Guppies.

For further information please read my article about the care of Mollies (of which the care for other livebearers is similar):
Aquarium Answers: Keeping Mollies in an Aquariums

Here is another good outside source for livebearer information:
A Livebearer Primer

Lake Malawi African Cichlids, female and male Kenyi, photo courtesy Eve In Cichlids some species, males and females are readily distinguishable by color, size or shape.
For others it is difficult or even impossible to tell the sexes apart in non breeding individuals.

Here are a few specific methods to sex certain Cichlids:

Here is a tip as to determining Mbuna African Cichlids from Jon V. (from "Everything Aquatic"):
"When you get a group of fry or brood, keeping them in their own grow out tank for about a month. During this time as you feed fry foods, watch the group to see who's getting the most of the foods. Even at that early age, I've seen a tendency which I think is noted in other forums, that males will even be aggressive for food as early as the first few days when released. After about one month of this, you can take a brood of say about 20, and the ones that have already grown out larger then the others, you have about a 75-80% chance, those are males. While I've seen on some occasions, females grow rather large early on, I find this has great accuracy when comparing the first and second months of age."


See this article from Jon V from Everything Aquatic:
Venting cichlids to determine sex

For more about sexing cichlids (from Ron Coleman’s Cichlid Research Site):
Cichlid Research- Sexing Cichlids


Sexing Goldfish is very difficult before they reach sexual maturity.
One way to tell is by looking at the shape of the vent. Females may have rounder convex vents while males have thinner concave vents.
The pectoral fins of males may be rather thick and stiff (compared to those of female goldfish) and with a more pronounced outer ray.

Another way to tell is by male goldfish will developing breeding stars on their gill covers and along the first ray of their pectoral fins when they are ready to breed.

For more about sexing goldfish:
“Goldfish Basics”


Breeding bettas embrace, BC Aquatics Male Bettas in the wild are short finned, so fins are not as reliable a way to tell the sex in bettas and is common belief.

Here are a few ways to improve the accuracy of sexing your betta:

  1. While looking at the male betta head on, there is an extra flap of skin under his body. The extra skin is an extended flap of skin from the gills.

  2. Again look head on, the males will have a much bolder head and fin structure.

  3. While a little less accurate, a male will generally not have a white dot under his body, in between his ventricles. Females will have this white dot.

Taken together you can give a much more accurate assessment of your Bettas sex.

See this article for more about Betta Habitat of which this knowledge may help in stimulating Betta Breeding behavior:
Betta Wild Habitat


Cousins to the Betta, in which both have a labryinth organ, which allows them to breath air directly. Both also come from SE Asia.

Many Males can be differentiated from females by the shape of the dorsal fin, which is long and pointed in males, compared to the females' shorter and more rounded dorsal fin.
Females that are prepared for spawning will show a pronounced swelling in the breast area, while the male will be more slender in the breast area.

In many Dwarf Gouramis, the males are much more colorful than the females (this is not usually true from my experience with other Gouramis such as blues).
Please note that these are generalizations and do not apply to all Gouramis.
Gourami sexing, male, female, dwarf

Currently I have not found any in depth articles about Gouramis (other than species specific) that I can recommend.
Either I will write one at some point in the future (as I did for mollies) or if one is brought to my attention, I will post it.

Other Recommended Reference Sites

Aquarium and Pond Information
-A useful source for current

Aquarium Lighting
Aquarium Lighting; Basic to Advanced

*Filtration Aquarium Filter Help, Troubleshooting, Reviews

UV Sterilization for Aquarium or Pond

A MUST READ article for anyone looking to move up from basic to more advanced fish keeping!!

*Ick; Ichthyophthirius Multifilis, Treatment, Identification, and Life Cycle

Fish as Pets with articles & commentary of Interest to the Aquarium Hobby
Including articles such as this one:
Melafix Dangers; Effect of Water Parameters on Melafix when used for Bettas, Labyrinth Fish

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

TMC Premium Fluidized Sand Bed Filters

Premium, second to NONE Aquarium Bio Filters, that with optional Oolitic Sand also maintain essential marine aquarium calcium levels, alkalinity, & electrolytes that are important to ALL Marine life, Goldfish, African Cichlids, Livebearers & more

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

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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

A good compliment to RO water or for any freshwater aquarium to add ESSENTIAL Mineral Ions:
*Wonder Shells, Mineral Block

For help with your Aquarium Chemistry:
Aquarium Chemistry; In Depth Information

World Class Aquarium Information
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*American Aquarium

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*PAR 38 LEDs

For economy HOB Filters that deliver the same or better results over filters often twice the price:
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*SunSun Reliable Submersible Power Head Aquarium Water Circulation PumpAquarium Power Head Pumps
Superior to Hagen or Marineland, yet more economical.

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

UV Replacement Bulbs/Lamps:
UV Replacement bulbs for Tetra Mini, Cyprio Vorton, Bioforce, moreG23 & G11 UV Replacement Bulbs (PLL & PLS)
*Standard Straight Tube UV Lamps
*UV Bulbs directory

The Best in Value UV Sterilizers:
*Aquarium & Pond UV-C Sterilizers/Clarifiers
Compact Aquarium and Pond UV Sterilizer, better value than Tetra, Turbo TwistCOMPACT UV STERILIZERS, Terminator 7 watt to 36 watt
*TMC PREMIUM High Dwell Time Pond/Aquarium UV Sterilizers

For Aquarium Treatment Products

*Aquarium & Pond Treatments
*Furan 2

Proper Pond Filtration, Cleaning, care, chemistry & Basics for maintaining a beautiful garden pond:
*A CLEAR POND; Care & Information

Fish Food Information:

Premium Canister Aquarium Filters:
Rena Filstar High Performance Canister Filters

Aquarium Decorations, Decorative Coral, Driftwood, DeCoral, PlantsDecorative Coral
Aquarium Decorations such as

*Plastic Aquarium Plants

*Aquarium Driftwood

*Aquarium & Pond UV Sterilizer Use Articles
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