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Corydoras aeneus - Complete Spawning Report

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[If you want a trip down a "NERM hole" . . . here you go!]

Spawning Report: Corydoras Aeneus (Bronze Corys)

We've successfully finished our 30 days with a small batch of corydoras aeneus (11 fry). Guess this will be the last BAP submission for 2020 for us. Learned a lot with this project. Took a year of growing the Corys bought from our LFS, and a solid 8 months mounting up failures to figure out how to raise them. We're raising up a larger batch of fry behind these (ca. 20-25).


(1) Eight months ago . . . setting up a dedicated Cory breeding tank

(2) Nov. 20, Cory eggs pulled

(3) Nov. 22, Cory fry free swimming (11 fry counted)
(4) Nov. 30, Fry growth and updates 
(5) Dec. 7, Two-week Journal - Chemistry Discussion
(6) Dec. 11, Update and New Batch of Fry following up behind Our BAP batch
(7) Dec. 24, Cory fry BAP graduation - adding fry (11) to adult breeding tank


24 December 2020




Corydoras Aeneus

Bronze Corys




Method of Reproduction: Egg Layer

Number and Gender Distribution of Parents: Eight Adults, Possibly only 1-2 Males

Origin of Parents: Tank Raised (Purchased from LFS)    

Approximate number of fry: ca. 11   

Date of Birth: Free-Swimming (11/22/2020)     

Number of Fry at 30 Days: 11




Spawning Tank Size: 20 gal. long

Spawning Tank Water Source: town / city water

Spawning Tank Water Changes: 30-50% 1x per week / bi-weekly

Spawning Tank Filtration System: Two sponge filters

Spawning Tank Temperature: 78-degrees Farenheit

Spawning Tank pH: 6.0 (water changes cause fluctuations up, but it always drops)

Spawning Tank KH: Unreadably low (tap water is ca. 4 dKH, ca. 71.6)

Spawning Tank Ammonia: 0 ppm

Spawning Tank Nitrite: 0 ppm

Spawning Tank Nitrate: ca. 30 ppm

Note: This tank has experienced a recent pH crash due to humic acid buildup from leaf litter decomposition and alder comes. We removed most the leaf litter and cones, and have been monitiring it’s chemistry.


Specimen Container: 1/2 gal. Lee’s large specimen container

Specimen Container Water Source: Bottled (RO) water

Specimen Container Water Change: 16-32 oz. / day until sponge filter added after 2x weeks

Specimen Container Filtration: None, just air line for 2x weeks; then small sponge filter added

Specimen Container Temperature: ca. 80-degrees Fahrenheit

Specimen Container pH — ca 6.0 (due to use of RO bottled water) and catappa leaf litter

Specimen Container GH — Hard to determine . . . very low

Specimen Container Ammonia: actually Ammonium - can be very high, 0.5-0.8ppm)

Specimen Container Nitrite: 0 ppm (after sponge filter added)

Specimen Container Nitrate: 40-80 ppm (very high)

Note: Specimen Container readings taken after period without water change




Spawning Tank Live Plants: Pothos (roots only), Cryptocoryne Parva

Spawning Tank Caves or Similar Hiding Places: Rocks from stream (boiled before adding), wood

Spawning Tank Substrate: Fine white stone (looks like coarse sand)

Spawning Tank Lighting Type and Timing: LED, 5,000 K, filtered through diffuser, ca. 14 hrs / day


Specimen Container Live Plants: None; only catappa leaf litter and alder cones

Specimen Container Caves or Similar Hiding Places: None until sponge filter added

Specimen Container Substrate: None; again, only catappa leaf litter and alder cones

Specimen Container Lighting Type and Timing: LED, 5,000 K, filtered through diffuser, ca. 14 hrs / day




Food Fed to Parents and How Often: 2x / day. Frozen Blood worms, Bug Bites flake food, wide variety of flake mix - Omega One, Kelp Flakes, But Bites Tropical Blend; live baby brine shrimp

Food Fed to Fry and How Often: 2x / day. Live baby brine shrimp, arctic copepod powder, sera micron, New Life Spectrum fry starter powder, finely crushed flake food




We couldn’t keep track of the number of times our Corys laid eggs, we collected, and they either failed to hatch, or failed to survive. The factors were manifold. We suspect there may only be one or two fertile males in the original breeding group. Many eggs were just not fertile. We also struggled with fighting off fungus spread in the hatching container. In the end, we were successful using two pieces of catappa leaf and a few alder cones that grow wild out on the edge of some swamps where we live. These release tannins, which help to fight off fungus spread. Also, the decomposition creates lots of context of microlife to flourish for baby corys to get their earliest source of food.


Before we were really successful we tried a lot of things to trigger spawning: heavy water changes; using rainwater / ground water for water changes; cold water changes; feeding a wide variety of foods; over-feeding / cleaning after; altering lighting plans; leaving lights on at night; addition of live plants (crypts); addition of a small Hydor powerhead for flow . . . and then we had the afrementioned challenges of trying to hatch eggs and keep fry alive.


Documented successes from folks in our fish club helped us, as we read some of their BAP submissions. It seems that if you’ve got a decent ratio of mature males and females, feed well, and keep their water quality up, they’ll do their thing eventually. We’ve had our adults for about a year before setting up their breeding tank, so, as with other catfish, they seem to take their time maturing to a breeding age.


Once we had fry, we changed out the water regularly with R. O. Bottled water. But once we were about 2-3 weeks in, our corys spawned again. So we started another batch but just used tank water instead of R. O. water. That batch is going very strong - about 20-25 fry — plus, we found 5x fry from that batch in the parent tank that must have been attached to leaves somewhere.


Some chemistry things we learned / are learning: (1) Raising fry in the little specimen containers is a stress on fish fry, and on fish breeders. Constant vigilance is needed to keep parameters from danger zones. Without filtration, ammonia and nitrite build up quickly. But with a sponge filter, nitrate builds up quickly as well. (2) Using catappa leaf litter and alder cones releases humic acid, and really makes pH dive. This keeps ammonia (NH3) buidup below the 7.0 pH threshold to ammonium (NH4+) which is _slightly_ less problematic for fish . . . but really not good in any way (3) R. O. Water, and our soft tap water, lacks buffer which prevents against pH crashes. We found that the pH crashed in the adult tank when we added some tetras, and kept finding them dead. We originally added a lot of catappa leaf litter and oak leaf litter along with alder cones to the parent tank. It was just too much, and as humic acid built up over time, the pH crashed — from 7.8 tap water to 6.0 (or lower . . . our kit really doesn’t measure pH lower than that).


Science aside, the jury is in: Cory fry are the cutest fish fry in the world! We all love watching them wiggle around. I think this species is one we’ll continue breeding and raising up for a while. Our fish stores are always happy to have some to sell. But we will probably not hold them in the grow out context much beyond 3 weeks so that they can be added into the larger colony and grow up in a better environment.

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21 minutes ago, Levi_Aquatics said:

your documentation of the process of breeding fish is great and very helpful!

Thanks! Glad to share. We ended up adding all of the cory fry together back into the 20 gal long. I think there's 40+ bronze corys in there now, all happy! 😅

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You might want to invest in a hang-on breeder box like the ones from Marina and Ista. They pump water through the breeder box from the tank which solves the water quality issues you had with the specimen container. Essentially your fish tank becomes a sump filter for the breeder box. The breeder box stays at tank temperature, there's a nonstop supply of fresh water into the box from the tank and the water from the breeder box then flows back into the tank to be handled by the tank filtration system. They're typically under $30 and work great. There are grids you put on the outlet to keep the fry in the box, and for really small fry, a piece of foam or filtering material placed over it can stop even the smallest fry from escaping. You just connect an airline and adjust the airflow to the speed you want water to move through the breeder box. Mine is currently moving two teaspoons of water a second through the breeder box. If the sound of the water gets annoying, you can place a short length of airline tubing in the outlet of the lift tube and aim it below the water line in the breeder box. It'll restrict the flow a little, but silence the noise.

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2 hours ago, gardenman said:

You might want to invest in a hang-on breeder box like the ones from Marina and Ista

You’re 100% right. Excellent plan for Christmas cash usage! Thanks for some tips on usage too. We never really heard about these until getting on this forum. 

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They're a great little box if you're breeding fish, or even if you need a "timeout" box for a naughty fish.  I've had mine since 2015 and it's a very handy device. Finnex makes one with a water pump, but I suspect that might be a bit too aggressive. You don't want the breeder box turned into a whirlpool. It looks like Fluval has taken over the Marina branding but the breeder boxes still look the same. An airline valve to adjust airflow and thus water flow is wise if you get one of the air-driven boxes. You could probably cut a piece of stiff foam and make a mini Matten filter near the water outlet by sectioning off an inch or so of the far right side of the box for extra filtration and to be sure no baby fish slip out. Mine's a medium sized box,, but they come in small and large also. I don't use the water siphon tube with eggs, but use an airstone instead to just circulate the water in the box and around the eggs. Once the eggs have hatched though I then switch over to the siphon tube to keep the water fresh and changing.

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