Lovely pics!
The point being that as with virtually all egglaying fish, the egglaying and the addition of male sperm has to happen at the same time. In other words, you have to have at least one female and one male (2 males better with corys) in the tank at the same time, and they both have to be "in the mood". This can be encouraged by conditioning them with lots of nice meaty live foods. You will notice when they spawn as they do a lovely little courting dance ending with male and female taking up the famous T-position.
This is different from the livebearing fishes where the male can sneak up on the female when she's not looking, and you can buy a lone female from the shop and find she's already pregnant. With egglayers, they both have to be there.
