You can put some Water Sprite (Ceratopteris thalictroides/ cornuta) in the display tank and the baby fish will hide among the leaves. You can then scoop the babies out with an ice-cream bucket and put them in a rearing container. You can use a plastic storage container or even a 20 litre bucket for a rearing tank.
Rainbowfish eggs are small (about 1mm diameter), round and clear when first laid. After a few days you can see black appearing inside the egg and after a week you can see the eyes and body.
The eggs have little filaments on the outside and these attach to plants or filter floss and stop the eggs floating off in the current or sinking to the bottom. The eggs sometimes stick to your fingers and a lot of people actually pick the rainbowfish eggs from plants and put the eggs into hatching containers.
If you use a gravel cleaner in the tank, you can do a light gravel clean just over the top of the gravel. Drain the water into a white bucket and let it settle for a few minutes. Then shine a torch into the bucket and see if you can spot any eggs on the bottom. I used to do this with some of my tanks before doing a water change. You suck any eggs off the bottom and leave them in a white bucket, then scoop out any babies that hatch over the next few days.
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To breed rainbowfish you want a tank that is at least 2 foot long. You can also use a plastic storage container to breed them in. Have a thin layer of sand or gravel on the bottom. Add a clump of Java Moss and some Water Sprite, or make a spawning mop from acrylic wool. Have an air operated sponge filter and heater (if it's cold) in the tank. Have a cover on the tank to stop the fish jumping out.
Put the fish in the tank, feed them, do water changes if they are in there for more than a week, and let them breed.
As mentioned previously, if you only have one species of rainbowfish in your current tank, just take some plants out and put the plants in a bucket of tank water. Leave them there for a week and see if anything hatches. Try to use a white bucket so it's easier to see the fry.
You can also put a spawning mop in your tank and leave it there for a week, then put the mop in a hatching bucket. Put a new mop in the main tank when you remove the first one.
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Just out of interest, you are suggesting egg yolk as a first food. Do you know why when I suggested that on another thread I was slammed and told that it was going to cause all sorts of defects in the fry.
Yes, I recommend boiled egg yolk as an emergency food for newly hatched fish.
Which thread was it?
I'm not sure why they slammed you or said it causes defects, it doesn't. In fact most liquid fry food for baby fish consists of boiled egg yolk and preservative so it lasts longer sitting on the shelf.
The only problem you have with boiled egg yolk is if you put too much in the tank, you can cause ammonia problems but that is it. It's perfectly safe for fish and has a similar molecular structure to fish eggs, and fish eat each other's eggs all the time.