- you need a male to fertilise the eggs!
They are egg scatterers - this means that the female swims about laying eggs all over the place - the male follows doing his bit.
they will normall spawn at first light so put the female and any male into the tank in the evening.
then the next morning see if she is thin again (or you should be able to see eggs in amoungst the marbles)
If they have spawned, take them out and leave the eggs in there.
after 3-5 days the eggs will hatch and tiny little fry will swim up to the glass and stick to it - they will stay there untill their egg sacks have been used. Then they will start swimming about.
Once they are swimming you will need to feed them.
I used a Liquid fry food for egglayers (liquifry no1) and then moved onto fine crushed flake foor as they got bigger.
You will need to do lots of water changes to keep the water good - or they will die. (Lots will die anyway due to nature taking its course).
You will also need to suck any debris (eg food and casualties) out regularly too.
after all that you should end up having a large amount of danios