Hello, just got back from a weeks vacation and came back to this. I have no idea what the dark spots on the sand bed are. Also, the slime-looking algae on the rocks. This tanks is coming up on 3 months old and I have done water changes biweekly now since the tank was cycled. Is this just a bad diatom bloom or is this something a little more serious? If anyone knows please let me know what I can do to fix it!!! Thanks!!