Many factors can influence where fish spend the majority of their time in a tank including temperature, water flow, other fish, preference and hiding places. Depending where the aquarium heater is placed, temperature in the tank may vary in different areas and fish may prefer one area to another.
Another reason fish stay at the bottom of the tank is water flow. Tank pumps can create stronger currents in some areas than others. It may be easier for fish to navigate along the bottom of the tank. Large fish, or different types of fish can also cause smaller fish to stay in one area of the tank. The bottom may have more algae for fish to feed on than other areas, and many fish prefer hiding in plants and tank decorations that are found along the bottom of tanks.