High stocking densities, high fish activity levels, large fish size and high water velocities all serve to increase the turbulence of the water and maintain any solids particles in the water column rather than letting them settle. This is particulalry important in raceway and D ended tank design, where the wastes must be transported to the end of the tank before they can be removed. Round tanks have a natural vortexing effect on solids particles, drawing them to the centre screen. This makes the round tank more self cleaning , especially where there are small fish involved. It is for this reason that they are in common use in hatcheries.
Some tanks are square with rounded corners.
Such tanks have the advantage of being more space efficient than round tanks. The slope on the bottom of tanks has an important bearing on the self cleaning ability of a tank, especially when water velocities and stocking densities are low. Round tanks are generally well mixed systems, whereas raceways tend to be poorer mixed systems with good water quality at the start of the raceway, which gradually deteriorates along the length of the raceway. If the whole holding volume of the raceway is to be used, then some fish will inevitably occupy the lower ends of the raceway, where water quality may be poor. This improves the mixing in the tank and provides a good compromise between raceways and round tanks.