Swimming, floating, sinking

Sinking

If the density of a body is greater than the density of the luquid substance, in which the body is dipped, it will sink to the ground. Stones and massive pieces of iron, for example, do not swim, they sink. In this case the weight force of the body is greater than the weight force of the water which was displaced by the body and therefore the weight force of the body is greater than the lifting force acting on it.
There are even some fishes who sink to the ground!

Not all of the fishes are able to float in the water. In case of many flatfish species (picture above) their swim bladder degenerates during their period of growth so they loose their ability to change their volume and therefore their density. Consequently their density is always greater than the density of the surrounding water so they stay on the ground of the water almost all the time.