Their turbidity current behaved very much like turbidity currents that form in the ocean. Turbidity currents are important to people because they damage equipment on the ocean floor. Also, turbidity currents are one of the most powerful mechanisms that move sediment to the deep ocean. If they performed the optional second part of the experiment in which they poured hot water into cold water, then they made a density current created by temperature differences. These are not common in nature, but they are similar to density currents formed by differences in the amount of salt dissolved in water.
Salt makes water denser, so the less-dense fresh water flows on top of the denser sea water.
I suggest that the students use a temperature difference to create a density difference in order to show that density currents can be created by completely different mechanisms. In the students' experiment, the hot water, being lighter than cold water, flowed across the top of the tank, though it did mix with the upper part of the cold-water mass. Eventually, the hot water mixed entirely with the cold water, and this was the end of the density current. This is what happens when clear river water flows into the sea. Turbidity currents and other density currents are very common in the world around us, and the students should be able to think of some examples.