I am pretty sure it has not happened overnight but I don't know how long they have been there. Since Christmas I haven't been able to fully open my door to the stand because a bookself had to be move in its way for the Christmas tree. I thougth I saw it there around Christmas but couldn't get in to take pictures, or close enough to remove it! If it were eggs what would be laying large eggs on the overflow? I have a guard over it so fish cant go down the overflow. This stuff is on the guard and the overflow. The tank has only been up since End of Sept but most came from the tank before I had to move it.