In nature you usually see just one sort of rock in a small section of river or lake. Choose either the rounded ones (best for a water setting) OR the angular ones (Suggests a scene above the water, even if they are under water).
Rounded rocks are rounded because they have been tumbled by water for many many many years. They are usually found in streams and rivers.
Angular rocks are usually found in dry settings, and are often used to make a scene in the tank like a hillside, with the rocks sticking up out of the soil. The flat ones also make good retaining walls.
A rock that is sort of angular, but also rounded may be found in areas where the water level of a lake rises and falls through many years. The rocks are not exposed to the water long enough for all the edged to have worn off, and not tumbled like in a river.
Look at pictures of the African Rift Lakes for this sort of rock.