Odd and metaphorical, I don't know what it means.

My brother had a new fishtank, with 2 unremarkable small fish not unlike slim guppies. The tank was 3/4 full of sand, with water above that. I found this unremarkable.

I watched them for a while.

When I came back a while later, I was astonished to see one of the fish point it's head right down, and swim down into the sand. There was a vertical channel in the sand next to the glass, like the entrance to an anthill. The fish swam straight down until it reached the bottom of the tank, and then turned another right angle into another small tunnel through the wet sand agianst the glass at the bottom of the tank.

I pointed this out to my brother and he explained that this species of fish did that. This explained why he had bought such unremarkable-looking fish.

The fish turned upwards again and swam, this time right through the packed sand. It wiggled and dug strongly, and reached the surface soon, righted itself and swam on in that unconcerned way that fish have.

I decided that I wanted a couple of these fish for my tank.

I came back a few hours later and saw several chambers excavated at the bottom of the tank against the glass. I saw a fish swim down into one of them, bite at the roof of the cavern, then swim back to the surface, where there was about an inch of water, and spat out several grains of sand.

I was impressed, both with the cool cavern-building fish and with the complexity of their behavior patterns.

I came back in the morning and saw that the result of evaporation and excavation was that the water level had met the sand level, leaving a tank 3/4 full of wet sand.

Two stranded dead fish lay upon it. Thier instincts had killed them.