First, it is useful to categorize female fishes into four categories: very productive, normal productive, and less productive, and infertile. In the case of a very productive fish, we do not need to do anything and the fish will lay eggs by its own everywhere and anywhere. Even when there is no male partner, it will lay its eggs. But most fishes fall under the category of normal productive, which will usually lay eggs in a comfortable environment with the presence of male partner. The problem arises when we have the less productive fishes. We need tricks to induce the spawning. In the case of the infertile fishes, nothing can be done. The infertility can be of two kinds: the fish cannot lay eggs, or the fish can lay eggs but the eggs cannot hatch. So, our focus is how to induce spawning in the less productive fishes, which of course the tricks will also applicable to the normal productive fishes.
Basically, I use two ways to induce spawning: to make the fish comfortable or to make the fish uncomfortable (or introduce changes). I will try the first method before considering the second one.
I live in the tropical area, so my methods might not work for other areas. Each breeder need to find his / her own ways in the two frameworks.
If we use biological filtration pond or tank, then the fish will be comfortable in a mature filtration. If we use water change method, then it is better to have a routine schedule. Male partners should be present. Outdoor or semi outdoor environment will be good since it is in accordance with the fishes’ internal clock. Enough sunlight. Introduce plants. Do not feed with growth pellets or tubifex worms. Feed with wheatgerm-based meal. Use live food such as frozen bloodworm and daphnia. Mosquito larvae is a superb food for this purpose. Normally, these actions will suffice to induce breeding. The fishes will mate at around 5 am. It is an advantage to be in the tropical area that I do not need to take into account the winter season or the water temperature.
For the less productive fishes, to make it comfortable, we can introduce a productive female one in the tank. After the productive one lay eggs, we can remove her. We can hope that the less productive one will follow to lay eggs.
Another way to make her comfortable to spawn is to put bricks or clay pots in the pond. Somehow it is believed that the smell of the soil can induce the fish to breed. I do not know how true it is, but I sometimes use this method combined with other ways.
If the comforting methods do not work, we might consider to make the fish uncomfortable by introducing change / surprise. Why uncomfortable methods might induce breeding? I read it somewhere that in the presence of death the fish might naturally think she need to lay eggs to preserve her descendants. So, the change might make the fish think she is going to die (smile). The mild measure in introducing change includes moving the fish from indoor to outdoor. Or if the fish is kept in outdoor, put her for several weeks indoor before moving her to outdoor again. We can also play with the water temperature. Try to keep the fish in a colder temperature for several weeks, then move to a warmer environment. Total water change can also be a surprise to the fish. Drastic water parameter change might be introduced also, mimicking the rain fall. Fasting (deprived of food) and then pumping the fish with good food can also be tried.
A more risky way that I sometimes use it to lay the fish in insufficient water level. This is the way I do it. As I make a total water change, when I fill the tub again, I put the fish in when the water level is still insufficient to submerse her. So the fish will be a bit stressful for a minute or two for lacking water, but will not be hurt / damaged. It is enough to make her think she is going to die for a while. This is the most extreme measure I take. But I seldom use it and I only use it for a hard case.
The breeding can also be induced by injecting substances such as Ovaprim to the fish. But I do not use this method.
These are the methods I use to induce spawning. My methods are simple. I am sure there are many more advanced and creative ways. I will encourage the readers to explore and to share their ways.