Well its possible that they could have been eaten, chemical warfare from a soft coral, or water quality... hard to say without knowing everything about the tank. But 9 times out of 10 they just aren't getting enough food, especially when they start to multiply and get into larger groups. I only fed mine once a week the first few weeks i had mine and they probably tripled. Then they started to shrivel up and disappear, so i started feeding them every other day and the population just keeps growing now with no recessions. So my guess would be lack of food, but it could be something else. HTH