In most cases, sexual spawning events are not due to stress. These animals must sequester the energy and materials needed to do it--it doesn't just "happen." Quite a lot goes into the process. Most operate on an internal "clock" of sorts, often spawning twice a year, IME, around spring and fall. Now, stress after the fact due to decline in water quality is a definite possibility. When animals like this have enough energy at their disposal, often they will devote it to spawning. IME, if they have it continually, the more often such events will happen.
