Spawning fish in tanks is clearly much different that what may occur in the wild. Breeders have a trick they will use with certain fish that are stubborn about breeding that involves the natural hormones fish may exude during spawning. In the wild these are quickly diluted and any effect they can have is similarly dissipated. But in a tank the water is limited.
This fact has led to a trick some breeders will use to get get similar species to spawn. For example, lets say you have a tank of panda corys which are spawning on a regular basis and doing well. But in another tank you have another cory species that refuses to spawn no matter what you do in terms of dry and rainy seasons, temperature changes, TDS changes or diet. What may then be tried is that when the pandas go into spawning mode, you take the water from the panda tank which should be full of the hormones associated with spawning and you put that into the other cory tank.
I am aware of this technique being used with a variety of species to include plecos and angels. Of course, the fish involved must have a solid genetic relationship. There is no guarantee that this method will always works, but it is successful enough that it has been used for some time.
The upshot of this is when one has similar species, such as two different corys in the same tank, this effect becomes automatic. The hormone effect will happen or not based on the fish and not on us. This would make it more likely that mixing certain species in a tank may increase the odds of them interbreeding. Think of it like a dog humping the leg of a human, the mating urge overrides the normal process. This is one reason when one works to spawn fish intentionally that most people do it in species only tanks.
The above illustrates something I have come to believe over the years I have kept fish. Nature has evolved fish species over a long period to be optimally adapted to their environment. However, as much of a guideline as that might be in a tank, it is not always how things will work. A perfect example of this are plecos. They have evolved to do well in flowing water, in many cases it may be very fast flowing. The fish will expend energy just hanging on. But when we work to spawn these fish in a tank, the need for big current is not what one might think. Just because the fish can function in high current does not mean they need it. In the wild there is no option, the current is what it is. In a tank we can change that. The fish still hangs on, but it expends a lot less energy on this task which leaves more for growth and reproduction as well as for fighting off diseases or parasites.
As a result there are plenty of successful breeders of plecos who do not have big current in their tanks. I know an expert Hypancistrus breeder who uses sponge filters in most of the tanks.They do create some flow, but nothing like in the wild. I can personally attest to the fact that the plecos which have spawned in my tanks have done so without huge flow. Similarly, one of the most successful cory breeders in the USA says he does nothing special to get spawns. Good water, good food and patience are his main tools. He has spawned over 70 cory species.
