Goldfish (and some other types, as well) suck up small rocks from the bottom of the tank and spit them back out again. Carp are bottom scavengers, and what they’re doing is looking for leftovers. That’s great, if the aquarium has gravel at the bottom. I use glass marbles though, instead of gravel. That doesn’t faze the fish a bit. They don’t care that the rocks they’re hoovering up are as big as their heads. They try to suck them up, anyway, sets off a ruckus of plinking as they root around in the marbles and try to inhale them.
Anyway, that’s what they’re doing in the two heads-down pictures. They’d just finished their supper, and were scrounging around for leftovers. Plinkety-plink-plink-plink.
red paper boat 