It does cause pollution, the spawning lasts for hours. It's like someopne dumping food into your tank all night. This is the 2nd event. Each one larger and messier than the previous. I log the dates and they run on exactly 6 month intervals. They fill the sump with white foam and an oil slick, the skimmate turns white and the bubbles have such a strength it lifts the lid off the skimmer. It forms like a white glue layer on everything. The smell is extremely strong oceany smell.
The event in itself is harmless, buit the resulting filth in my otherwise clean tank has both times started a brown jelly infection on one coral last time, and 2 this time. So with water changes, strategic fragging and tons of ozone I have not lost anything yet.