This has happened before and I never really noticed it but I just had a spawn event happening yesterday. Its pretty cool. You can see all the snails bunch up together near the surface of the tank along the perimeter of the tank.. There were 3 groups of them. The all of a sudden milky sperm like liquid comes out of each snail. This lasted for 30 - 40 minutes and everyone went back down and started spreading out. Its pretty cool. So todays date is 9/3/09. I'll update if I see any surviving baby snails. The last time this happened I had a lot of baby snails in the filter sock (I need to change them more frequently!).