This article was original and not written by AI. It’s from memory when I bred Ranchus in Vet School.
So, the first thing is: Goldfish are horny and they’ll breed without assistance. But you’ll get mutts. Survival rate of fry will be low (ish) and it’s no fun.
The babies won’t be of value either.
When Koi and Goldfish cohabit: All the babies are goldfish babies because the Koi don’t care to eat goldfish babies but the Goldfish LOVE Koi eggs.
When someone has Koi and goldfish together and they say “Look at all my Koi babies” you can safely say “Those are all Goldfish babies unfortunately.” Maybe a few hybrids. Which are priceless.
They’ll protest because they saw the Koi spawning and laying eggs. Understandable. But the attrition of the Koi eggs and fry is nearly 100% around goldfish.
But let’s say you wanna breed goldfish of tangible value like Orandas or Ranchus, under controlled circumstances so you can control the parent genetics and get the highest possible fry-survival rate.
You need male and female goldfish of at least 4 inches in sufficient space (55+ gallons) with very good water quality and plenty of aeration / circulation / surface disruption.
They need to be fed heavy. And they need variety and high quality and they need protein. (They’re reported to be plant eaters and they are like people eat salad) they still need bloodworms, brine shrimp, beef heart, freeze dried Krill.
Every once in a while, sometimes more than twice a year, the femal goldfish will get larger than usual with eggs and one morning, (almost always morning) the fish will wake up and one or two fish will be “running” the fat one.
This won’t happen unless there’s some sort of SOMETHING for her to swim into like a plant, a space between the heater and the tankwall, an ornament. A soft fake plant is enough. A mat of live plants or floating Wisteria is lovely.
The males (and some females) “run” the female around the tank and in her haste, and eswpecially as she gets “caught” in a confined position, her “abdominal press” or her struggle will cause eggs to be expelled from her cloaca.
At the same time, the male produces gametes in the same fashion, “pressing” the female in the plants, matting, washcloth.
The problem is, the eggs that get fertilized in the plants or matting will STICK to said plants. Only fertilized eggs exposed to water will be sticky.
If you have loose eggs “”blowing” around the tank bottom, they’re not fertilized or they would stick to the glass.
Ideally, the fertilized eggs would be removed to the safety of another tank with high aeration and a stable temperature and WITHOUT parents and other adult goldfish to eat them all up.
So what I did was when I discovered the female being ‘run’ by the other males, I’d take her out and put a few drops of water on a plate. A little puddle. Then I’d take that female and hold her over the plate and gently VERY VERY gently massage her abdomen from the top of her belly bulge near the dorsal fin, downward, and back towards her vent. If she has ANY eggs at that point they will slither out.
Spread the eggs out in the puddle. People use feathers. I used a piece of plastic fish-bag. You don’t want to break eggs while smoothing them out.
They won’t stick yet. And be careful because she will periodically kick her tail around and if you’re not careful she’ll either knock your plate over, or just scoop eggs off into the room.
Grab the male and do the same thing. Massage the abdomen, taking care his tail doesn’t wreck your project.
Water will run down the tail onto the eggs. No one cares.
The milt (his “stuff” slithers out and pools on the eggs in the puddle. Again, use the feather, or little piece of plastic fish bag to mix up that whole shenanigan and try and get the eggs in one, thin layer on the plate.
Two eggs on top of each other can’t “breathe” well. Water circulation is less-good and fungus is more likely.
You gotta wait a minute before you do anything with that plate.
You should kinda rock the plate around a little until the eggs stick. It can take a few minutes.
Then put the plate in a private tank with water from the parent tank, well aerated, and about 76 degrees, whatever the parents were kept at.
If you want to use some Methylene Blue to minimize fungus on the infertile eggs, that’s fine but wait at least 4 to 6 hours before you do it. It’s spermacidal. If you put the plate right into Blued water the egg hatch rate will be worthless.
Fertile eggs rarely if ever fungus unless the water is “new” or “uncycled” or poorly aerated, or too cool.
But that’s only because the eggs died. Not because the fungus was so “bad ass”.
Depending on temperature, the eggs will hatch, if I remember correctly, in 36-48 hours and the babies summarily fall off the plate and into the gravel or marbles on the tank bottom.
They lay there like they’re dead, for 24 hours living off their little yolk sacs.
Then they stir to life and swim up into the proper limnion in search of tiny food.
Sponge filtration is practically essential unless you have SUCH a balanced system for the fry that they need no filtration and have good water from a balance of live plants and aeration.
Feed the fry heavily. Aerate well except the fry should not be buffeted around the tank. They are poor swimmers. And if you’re not careful they will tire, fall back into the gravel and some will not be able to get back up and out.
Feeding fry used to be all about boiled egg – you’d boil water and then (using a milk frother) you’d pour the scrambled, raw egg into the boiling water while frothing it. The egg boils like a gross yellowing grain on the water surface. Take all that shenanigan off the heat and then pour it through cheesecloth and you have enough food for that batch of fry. Gross.
‘Thing is, nowadays you can just buy a paste of food that you just dribble into the tank and voila!