I found this blob on my sand this evening. I have no idea what it can be. It seems like some sort of eggs in an enclosure. It's soft (waves in the water) and small (1/4"). It stays directly on the sand...
I'm not sure what it was but it's gone. It either hatched or it was eaten. I didn't notice lots of little somethings in the tank (although they may be planktonic or hide during the day). I'll post if I'll suddently see lots of little somethings.