Since it is well known that adding cooler water encourages many egglayers (such as cories) to spawn, I think we may deduce that the coming of the rains does bring about some sort of change of temperature in the wild, though it may not be enormous. Then again, the temperature drop from adding 20% of tap water is not likely to be that enormous either; it hardly comes out of the tap at freezing temperatures.
Hi dwarfgourami,
True, the addition of cool water often brings about spawning in corys, but this is only one of a number of things that is going on in their natural environment at the time. It happens annually too, not every week. During this time the corys that were hatched the previous rainy season have reached maturity and, along with the older corys, they have eaten their fill during the year and should be in prime condition to face this event. Not only does the water temperature change, but the chemistry changes too, and the availability of food is seasonally different.
On the other hand, sudden drops in temperature have been known to set off bouts of ich in fish that can be effected by it, and sudden rises in temperature is one of the stress factors that sometimes lead to bacterial infections. While a change of just a couple of degrees is probably OK, I wouldn't want to take a chance of varying the temperature any more than that.