Ok, one of you jokers on here needs to own up! ;Shifty
You know when you look at something and the only reasonable explanation is that someone had their hand in the tank?!
This shelf is at about mid depth so 8 or 9 inches up and the only way I can think it got there was if someone took a pinch of my sand and sprinkled it there just to annoy me!
Engineer goby? No, too high up and it would be more sand from one spit!
I do know what did it I think, the tassel file fish does take mouth fulls of stuff when it grabs something juicy and then filters the sand and crud out, took me a couple oh hours thinking about it to figure it out!